Expect Marriott to eventually clean up unearned years of LT status, very likely by Jan '19. Of course it's possible their IT are simply too incompetent to sort it all out, but I'll be very surprised to see Marriott leave what anecdotally appears to be millenniums of unearned years on the table. I fully expect thread after thread to be kicking off around these parts any day now when Marriott begins to crush the LT dreams of those whose balances have been artificially inflated (possibly including yours truly since I'm sitting on 12 years, probably of which at least 10 are legit but it's impossible to know without meticulous record keeping on my part).

I would absolutely consider diversifying. Marriott status probably has nowhere to go but down over the long haul, so you'll want a balanced portfolio of status cards in your wallet. You may only stay in major cities right now, but careers have a way of taking unexpected twists and turns. Besides, the more places you have status at, the more options you have when it comes time to cash in those hard-earned points. A nice status-based upgrade can really make all the difference during a romantic vacation or whatever, and Marriott isn't always going to come through with a nice property with points availability during your exact dates of vaca.

Having said all of that, if you prefer the Marriotts where you're currently traveling for work, I don't know how far out of my way I'd go just for the sake of diversification alone. I tend to find my sweet spots and return to them often so long as they have availability at a reasonable cost.