on simplex there are not thousands, but hundreds of unofficial servers. tho you're correct about putting a backdoor into a one mainly-used-by-everyone client is easier, than into a lot of different clients. yet, this has also another side: if the code is open and has only one version, there's more eyes on it than if the eyes are spreaded onto a lot of different clients. it also backslashes: if there's a lot of clients, CIA can make its own — taking its budget into account it will be even easier to make a good looking honeypot than to insert a new backdoor into an already watched app. "just use the serious clients"? yeah let's limit clients to only one trustable, like simplex did.
also, the government funds lots of shit: tor, tails, signal... doesn't mean they're automatically becoming bad apps becos of that (but I bet they hope we think so)
I'm open to change my mind if I'm wrong, so please answer, if you disagree