The average SL user only has one body, only the top % of shoppers have multiple bodies because the average user is not willing to deal with the headache of changing bodies constantly and the price of it.
From my perspective if the new body does not support older clothes then we get into a split situation. I can either switch to the newer body and lose all my older clothes or I can stick to my old one and watch creators support the newer body only, which leaves me with no new clothes or a reduced amount.
You could argue you could switch between the bodies, but this both a major inventory management nightmare but also you can't mix and match things. Most outfits I get are that, just a dress or something, they don't include stockings/socks/shoes/other accessories. So for example if I buy a new Lara X dress which does not fit the current Lara but all my socks and things are for the current Lara, then it simply won't work at all.
The only way for this to work is if I abandon everything I have, somehow everyone magically updates their things for Lara X which is impossible since I have things from years ago, the creators have left SL or Lara X works with older clothing.