Window replacement in modernized old buildings

The SFP doesn’t matter to us at all, I only use it to get the U-values. The (obvious) window and door replacement is also the only measure we are taking. I am very grateful for your answers but somehow the thread went in the wrong direction and unfortunately none of my questions have been answered or I don’t understand it. As already mentioned, the SFP is irrelevant and I didn’t ask any questions about it. Maybe my question wasn’t clear enough and I’ll try it differently:

A fictional standard multi-family house has 4 residential units and a stairwell. The apartments are all insulated from the inside (with a U-value that allows triple-glazed windows). The stairwell was not insulated and has a U-value that allows double-glazed windows. All the windows are from 1970, so very old. The residents like to keep their front door open for a long time. The windows in the apartments will definitely be replaced with new ones. In this case, would one also replace the windows (with the best possible, i.e. double-glazed) or does that make no sense from an energetic or building physics perspective? Is the heat loss, for example, negligible since the heating is in the apartments and they retain the heat? Or is it even counterproductive because it promotes mold growth, since old windows provide a constant air exchange? Should all windows (apartments and stairwell) be kept at the same level (e.g. all only double-glazed) or can they be mixed?

Questions in a block paragraph are hardly noticed because they do not stand out. I also do not believe that enough construction experts are here to answer these questions. I would, if not already done, visit another forum where more experts gather. Maybe then it is more of an insulation issue?

The apartments are all insulated from the inside (with a U-value that allows triple-glazed windows). The stairwell was not insulated and has a U-value that allows double-glazed windows.

All U-values allow both double and triple-glazed windows. Even in completely uninsulated old buildings, triple-glazed windows are no problem at all with proper installation (keyword reveal insulation) as well as proper heating and ventilation behavior.

We don’t care at all about the SFP either, I only use the U-values there. The (obvious) window and door replacement is also the only measure we are taking.
I am very grateful for your answers, but somehow the thread went in the wrong direction and unfortunately none of my questions have been answered or I don’t understand them. As already mentioned, the SFP is irrelevant and I did not ask any questions about it. Maybe my question was not clear enough and I’ll try it differently:

I underlined the crucial part in the quote.
Your question was clear enough, and I understood it immediately. You didn’t ask any questions about the renovation roadmap, and it is only irrelevant insofar as you recognized large parts of it do not match your priorities. But the consequence of underrating it is wrong. Use it as a structural template for creating your own plan. Because you will need a plan. The attitude “the only measure I am targeting in my first step is known, I don’t need a proper ‘plan’” is, from my professional experience, wrong and would cost you about 30% more in total (or almost 25% slower / less productive) in the long run. I have already written you proposals for the structure of your individual plan (very important: besides the columns “measure” and “budget,” it must also include the time!). I mean every differentiation of the window-related measures completely seriously. Don’t forget that you are being advised here by a former window manufacturer. Full replacement (referring to individual windows) as the only measure is nonsense, and full replacement (in the sense of all windows) even more so. That would only literally realize the saying “throwing money out of the window,” nothing else.

Or is that even counterproductive because it promotes mold growth, since old windows ensure a continuous exchange of air.

Even the commonly ridiculed rebate ventilation here still makes more sense than simply leaving old windows in place. That is NOT a relativization of my recommendation not to shoot every sparrow with a cannon!
A “colleague” in the field of homeowner education, whose videos I can highly recommend to you, is the “Energy Saving Commissioner” (provided you are not allergic to southern Hessian dialect).

Our house from 1921 was insulated with 6 centimeters in 1993, I don’t have the thickness of the walls at hand right now, but definitely quite thick. Except for the basement, we have had triple-glazed windows everywhere for 7 years (previously double-glazed) and have had no problems at all. We already ventilate generously for comfort reasons. Regarding the question about the window in the hallway, I would probably not install a completely airtight window there or one with a ventilation rebate or something of the kind. But that is just an amateur opinion.

Is the heat loss, for example, negligible because the heaters are inside the apartments and they keep the heat. Or is it even counterproductive because this promotes mold growth since old windows ensure a constant air exchange. Should all windows (apartments and stairwell) maybe be kept at the same level (e.g., all only double-glazed) or can you mix them?

Relevant is the thermal envelope of the house, which should also be determined during the building survey and the creation of an ISFP. So a continuous "line/area" around your building where the heat is supposed to stay – to put it simply.
As a rule, it makes sense, and this is also the case here, to include the stairwell of the multi-family house. So you can ignore the airtightness and thermal insulation of the apartment entrance doors and focus on the losses from the stairwell to the outside air.
So: corridor included in the thermal envelope -> windows should be good. The wall here also allows triple glazing.

To avoid mold growth / moisture deposits in the only indirectly heated stairwell, you can do various things.
Advantageous: window rebate vents, with the disadvantage of uncontrolled heat losses and practically a (partial) cancellation of the benefit when replacing the windows, but with virtually €0 additional costs.
Also possible: heating the corridor with an additional radiator in the stairwell with fan assistance. This way you have no temperature gradient between apartments and corridor and at the same time air circulation. Of course, the piping must be suitable.
The overall heat demand of the building will not increase significantly because the heat amount is no longer provided by the apartments.
Or: ventilation with heat recovery in the corridor (a device about 30x40 cm with 180 mm wall penetration) and thereby a drying of the air without additional heating. Probably costs about the same as the radiator but should be eligible for funding in connection with the window replacement.