I cannot express enough how terrible an idea I think this is, nor how strongly I feel that way.
This is a great way to lose every single group (okay, maybe just the vast majority of them) that is currently playing above 10th level and would like to convert.
For God's sake, there is no reason to release the game in an incomplete form.
If I run a high level game with gnome druids and half-orc barbarians fighting frost giants, there is no reason why I shouldn't have everything I need to convert on day 1. NO REASON.
The whole crappy "let's save some mainstays for later" approach pissed off a ton of folks about 4e. You COULD NOT play a gnome, half-orc, barbarian or druid on day 1... in fact, you had to wait for a year.
Likewise, if you wanted to use certain classic monsters- REALLY CLASSIC- like frost giants, gold dragons, ankhegs, etc., you had to wait a year.
Let's not. Please, let's not.
Don't hold back, tell us how you really think.

Do people still convert ongoing campaigns from one ruleset to another? Every time I've done it, or seen it done, the campaign has crashed and burned, so it didn't occur to me this would be an issue. It seems far better to stick with your current ruleset and convert material at the GM's side.