I'm not 100% sold on that being true. I think people accept or reject new ideas based on how well such ideas conform to their values or preconceptions or self-interest.
You're talking about the reasons people change. Doesn't change the fact that they do.
I'm not sure Thingol, before his meeting with Melian, could have been called mature. I vaguely remember that in Valinor, there was a set age after which Elves were considered mature enough to marry, and that age is in the centuries.
As for Thingol himself, he wasn't majorly contributing to the sundering of the Sindarin and Quenya languages. He was entrapped for centuries staring at her. By then, the Vanyar and the Noldor had departed, and Tol Erresea had returned and taken away his brother and those of the Teleri who followed him. The Teleri who remained had already started shipbuilding, IIRC, and their exploration of the seas is bound to have changed their language a lot. Elwe probably picked up the changes, and in the millenia between his return to his people and the return of the Noldor, the languages bifurcated even more, and all that makes perfect sense. A lot went on, both in Valinor, where we know some of it, and in Beleriand, where we do not. The absence of much story doesn't mean nothing happened. Certainly, enough happened in the form of many of the Sindar having kids, including the birth of the only Elf-Maiar child, Cirdan gaining the favor of Ulmo and Osse, and their shipbuilding and port building reaching new heights.
But most of those events happen after Sindarin had evolved. Men woke when the Sun showed up, and that happened around the time the Noldor arrived in Middle Earth. The Noldor came in hot pursuit of Morgoth, so even though he had time to initiate the Wars of Beleriand before they got there, it could not have been long enough for the Sinder to change.
I'm not sure where you get the idea that Sindarin evolved to one point and stopped. The word for sun, for instance, is different in Quenya and Sindarin, which makes perfect sense, since they had no contact during the first sunrise.
And once the Noldor did come, Sindarin certainly got a bunch of loan words, which were adapted to fit Sindarin, but share more roots with Quenya. And the Noldor themselves picked up Sindarin, which would have also added to the change. I do not think the idea of a done and dusted Sindarin existing at some point makes sense.
Whyever not? The Dwarves of Belegost built Thingol's capital in Menegroth, and some even lived there in an embassy. It makes no sense that all this was achieved via sign language or something, so clearly, there must have been linguistic influence both ways, though if I remember right, the Elves were unable to pick up Dwarven.
This just isn't true.
Uhh... this is utter nonsense. Language can change without your regard for bloodkin. You do not need a fundamental personality shift for language to change. We cycle through new words and phrases all the time without suddenly finding ourselves not caring for friends or family, and we only live for 80-100 years.