The basic foes are quite nice for my needs as they are in the MM right now. If I need to make a change it is what? A minute or two? A spell selection here and there and voilà ! It is done.
But with this new stat block, I will need to take the DMG and adjust things, retrofit the whole stat block to respect both the intent and the appropriate CR.
Jist take Vecna, at the base it is a caster, a lich. Now I need to add spell slots, choose the spells and new abilities and compare these new abities to the current stat block and the evaluate with the guidelines in the DMG if I did a good approximated job.
With the classic stat block, if a spell is not to my taste, I simply pick an other one of the same level and I am done and good to go. Whether you acknowledge it or not, for those that play with the MM and 5ed starting design, this is a huge change. Much more drastic than the alignment change they attempted a few books ago.