Floor slab - basement insulation only on the inside for KfW70

Hello, today a structural shell builder suggested to me to insulate the floor slab of our basement only from the inside, meaning not to install XPS under the floor slab. No one knows how the material will behave in 20-30 years, he would only insulate from the inside – the internal thermal envelope would then also be easier to close.

Is there any truth to this or does nobody build anymore without external insulation?

Regards
Pit

So the same was also suggested to us. Having [Bodenplatte] is simply more expensive and insulating inside is enough.