Look at what they did for the last 20 years for Basecamp and other product. It doesn't seem very likely that they randomly raise prices for existing customers.

They support MBOX exports and have expressed that they will support custom domains. Between the two, it’s hardly lock-in; you can export as often as you like and keep your address.

Point with the whole Hey thing, is that they've gone a step beyond the client of an email server. And that's the beauty of it. I am all in for Basecamp, and will definitely try Hey, as email has gone from worse to terrible for me over the past 5 years or so.