There is very little chance of them taking this back. At best they will put into place a few logical restrictions, maybe an approval system.
Well odds are that Modding will revert to the "old" way where addons are hosted on a website and manually installed.
ex. before workshop everyone who used gmod used garrysmod.org for addons
What valve will really be upset about will be when people start "pirating" paid mods, and that's just causing a cluster forget of problems because then eventually there would be an anti-piracy system and I believe that would add restrictions to modding because maybe mods would have to be "liscensed" by then.