No substitute for decent code

Wordpress 5.5 gave us lazyload which was most significant performance improvement (especially on mobile) I have seen since 2011.

…if this thing really is a good idea why is it not in core ?

The idea that you can take CSS and JS bloat and solve the issue by bundling and minifying is misguided and will cause issues later on. I was initially excited by the idea and the page scores but soon realised it was just making everything too complex and making my sites more fragile and harder to maintain.

….and creating mirror images and a lot of rewrites to the webp version ?

If you really want this sort of thing there are many better alternatives available… be guided by the reviews.