This part I disagree with. After the TBC: Classic is live, each account that has 1 level 60 toon on a classic server on it should be granted either a free copy for their 60 to the new TBC server, or make it a purchaseable 1 time use item to copy a 60 to TBC.
This way you still have your 60, and you can continue leveling to 70. Later same thing for wotlk.
This is very authentic. Each time a new expansion came out players simply leveled their character that was already max level. Some people changed mains and started over, but the majority of players simply leveled their already max level mains.