You don't mention this in your criteria, but do you plan on doing maintenance and some repairs yourself, or will you be leaving it to the dealer and shops? If you plan on doing any oil changes, trans fluid changes, diff fluid changes, etc. yourself, scope out component/access/drain locations before hand. As a rule of thumb, Infinitis are easy, Acuras are okay, and Lexus can rot in [censored]. Kind of surprising, but as simple as some Toyotas are, I find Lexus vehicles to be an all around pain in the [censored] waiting to burn you, cut you, scrape you, or generally be aggravating. Infinities are pretty service-friendly though, I like them. I too would have major reservations about the steer by wire in the Q50 though. It has a safety feature so that you should have direct steering if it fails, but I can't imagine the driving experience to be that wonderful with it. Maybe numb, but probably not good.
I can kind of see where brand growth would be a concern for resale/service, but probably not in a 7-10 year time frame, even if you wait a couple years to get the car. Parts will be available for any Infiniti or Acura 7-10 years down the road. People will still pay good money for clean ones, even if the brand shrinks. Not a good reason to pick a Lexus instead.