I think you're setting this thing up in a way, where there's no way to defend having them.
If you mean by "quality depth" someone who is instantly ready to step into a starting role (preferably also proven), you're right. Veeeery few UDFAs can provide that. Out of ours, I'd count Kuhn and Pennel as such. Tolzien and Gunter likely can't replace the starter, but few could.
But TT and MM don't count on them being starters, either! Mostly you can only get role/ST players outta UDFAs. They can be quality depth at that specific role. Most of them are deep depth, who rarely play. Last guys on the 53, with maybe some potential in the future. And are dirt cheap.
If I have to guess your reasoning, it would be that we should replace some UDFAs with UFAs, who are probably more impact or provide depth of better quality. But I don't see those two as interchangeable. Those last roster spots better be cheap and play STs. I'd look more into the middle-level contract guys as those who could be replaced with better and richer alternatives.
(Sorry in advance for the guesses. I tried to make this conversation move somewhere.)