Cone drivers break in, of that I am sure. Fs and Vas will change, and some other parameters. I've measured it, so have many folks I know. I can provide measurements, if you like.

You're right tho that they do kinda plateau. Once they get loosened up, then they don't change a lot after that. It's no big deal. The surround and the spider flex and get more supple, that's all.