It is just a guideline.
What does one do after about 10 years... paint outside, paint inside, new kitchen, possibly repair the roof, redesign the garden, photovoltaic on the roof, there are many things, and if you consider that prices almost double in 10 years, then the 36 are only worth 18. They disappear quickly.
that is really interpreted more than pessimistically. first of all, all the renovation works you mentioned are EXACTLY what you save for every month. furthermore, many of the mentioned items last significantly longer.
- painting outside: for brickwork complete nonsense or not possible at all
- painting inside: sure, it has to be done, even earlier. but you don’t do it all at once like with a new build, rather gradually, and it’s a great thing to do yourself. renovating one room does not cost the world
- new kitchen: no one buys a new kitchen after 10 years if they buy something reasonable from the start. of course, if you are swimming in money, you can do that, but for everyone else the kitchen easily lasts twice that long. individual appliances have to be replaced at some point, but a good oven doesn’t cost 1000€
- roof: seriously after 10 years? if anything, it would be storm damage, which the insurance pays for
- outside area and photovoltaics: voluntary things that don’t really fall under the expenses for such savings
the mentioned guidelines are largely good. you should definitely save something, but less as a monthly amount x and more as a reasonable fixed sum. of course, if you don’t have 1000€ free every month by strict saving, you have to have more. but what EVERYONE must have ready for the house is enough money for an extensive repair of the heating system. as soon as the warranty runs out, that part can quickly cost 2-3000€. and then the money MUST be there immediately, especially in winter. depending on how quickly you can replenish a certain amount, you need to have different amounts saved. I personally consider 5000€ as an iron reserve just for repairs on the house sufficient, since we could replenish it very quickly in an emergency. in addition, we have another 5000€ reserve for "payment defaults", so in total we save 10000 for house + repayment. everything above that goes into the next car or special repayments.
the return you make today on special repayments is actually quite easy to calculate. just take the remaining repayment period and multiply it by the interest rate. 1000€ special repayment at 2% interest and 20 years remaining repayment time is 400€ (20x2% = 40% -> 40% of 1000€ is 400€) saved interest or 2% return over 20 years.
so if you can invest the money somewhere at (or above) your interest rate, it's more profitable than special repayments. if you invest the money yourself, depending on the investment type, you also have it available in an emergency and can, for example, simply cover the repayment rates with it. if you repay 100,000€ extra over the years but then cannot repay for 2 years, the bank will be on your case (maybe you can do something about that with the many special repayments, depending…). if you have the 100,000€ in your account and use it to cover payments, the bank doesn’t notice anything at all.
when the house is 10 years old, we will start saving then, but much more aggressively. for now, we will first repay the extra money that is left over.