So, of all the concerns that exist, only one of them is on the football field (we know what we have)? That isn't a snarky question, by the way. I think salary cap/locker room stuff is important, too. But they have oodles of dollars to spend, the contract is probably very similar to every other "large" contract handed out in the NFL (as you noted, he very well could be expendable in 3 years), and the Patriots already have a stigma of not paying for their players (Spikes, Jones, Seymour had all made gripes about it after leaving). If paying for guys like Mankins, Mayo, Wilfork, etc didn't move the needle on that stigma, I'm inclined to think that signing a slot corner for a few years wouldn't help.
On the flip side, while we "know what we have" in Logan Ryan...we also know what we have with Logan Ryan. He's a slot corner that can't play on the outside - which BB is willing to pay for (Kyle Arrington), but not at $10m a year - and needs some help from the defensive system he's in due to lack of speed/agility. He's also a very heady football player that can make great plays and who was a huge part to the success of the team last year.
While we don't necessarily know what we have in Gilmore, there is a feeling that he can be a true shutdown corner and has the ability to play all over the field while doing so (Adam Mays and Mike Lombardi talked about this in passing on The Ringer a few days ago prior to the signing). For an extra $3m a year, I take that every day and twice on Sunday.