Switching to POS will mean a lot for Ethereum, a lot of miners will just mine something else (ETC?). What this will give to Ethereum nobody knows, I can suppose less miners = less transactions, less transactions = less market cap, less market cap = less support walls, less support walls = crush of the price. Please note that I don't have a degree in economics, it is my humble opinion.
you actually have it wrong, it wouldn't be less transactions, a lot of ethereum would be locked up so there would be less big transactions but more smaller transactions, the market cap would go up insane because there's less supply, and less supply means higher demand. the problem with it is people can then just lock up their ethereum and get paid in pos and just sell their earnings from pos for ridiculously high prices so while the rich get richer for doing nothing the people actually using ethereum for smart contracts end up having to spend stupid amount of money and fees. while all the people that have their ethereum locked up have to do is sit on their greedy fat ass's while they get paid for doing nothing