Making in progress joining even more of a crapshoot than it already is won't exactly help the problem much. I'm sure someone at square will feel nice about their lil' half-torn bandaid fix, though they'll probably be the only one.

I present you two solutions that make more sense:

A) In-progress queues only for in-progress, the up-side is for people starting a fresh run they won't half to deal with constant queue drops from people fishing for in-progress. Long term, may lower player pool population for fresh starts as it would most likely be viewed preferable to queue in progress, though many CT groups form a full party and then queue up anyways.

B) Change the way loot works for CT and future weekly loot instances. Require a full clear of all bosses whereupon the player is rewarded with a token they can exchange for any of the pieces of loot that drop from that instance. Remove actual boss drops. May also lower player pool population as the week goes on, as it would only need to be cleared once per week for an item, and then spike again on monday.

I'd personally prefer both be implemented, but either one would stop the current problem. There's probably no actual perfect solution that makes everyone happy, but there's still plenty of ways to improve things without pissing too many people off.

Spoilers: the current proposed change is not one of them.