A7III and A9II are the first good mirrorless that are finally pro-grade.

Camera aren't terribly expensive, but if you have 8 - 10 pro grade lenses which will cost on average $2000 to replace with E mount variants that might sound expensive. Particularly if you don't produce more money by switching to mirrorless and buying those lenses.

Yes, the existing lenses out in the field certainly kept Nikon and Canon in their position. Anyone with a large amount of lenses would stronly prefer to upgrade to a compatible camera.

This is what makes their move to mirrorless so tricky. While adapters for the existing lenses exist and work, in the long term you would want to upgrade many of your lenses to their mirrorless counterpart. Especially, if most new lens designs are made for the mirrorless lines.

A7ii isn't quite pro grade but if you want to spend $1000 or so for a full frame camera Canon and Nikon don't come close.

I got sick of the kit lens so I got a cheap used 50mm which can shoot tack sharp and I just unboxed a 90mms macro from the G series and will try it out today.

Is there any way to adapt the old lenses to the new body style?