• The alternate accounts are sure to stop if there are no more signatures.
• Buying of bitcointalk accounts will cease to exist as the buyer has nothing to gain from it.
• We'll have less number of post and also traffic, because most people tend to make many post only because of their signature campaign but we still have a few who are not in a signature campaign and yet are actively participating in discussions here.
• As for the spam and sanitization, I don't think removing signature will completely eradicate all because even without having signature you can still find some trolls here in the forum but it's sure going to reduce it drastically.

For the first few months of the signatures removal we'll still notice high participation from users but with time it's slowly going to reduce and only those that are truly interested in the development of Bitcoin will be left here in the forum.