Some people are just irrationally biased against mergers - on my last flight, the couple next to me could not stop blaming the merger for us leaving SFO 5 minutes late, even when we arrived at SEA on time. It was Friday at 7PM, heavy travel time for SFO-SEA.
I try to fly Alaska everywhere too - never had a bad experience, on-time every time.
The oldest part of their fleet are B734's of which they have 5 (plus I think some Combi's which only service Alaska) all of those are due to be gone by the end of 2017. The rest of their fleet is B737/B738/B739's - all but 30 are B738/B739's.
In fact among legacy carriers Alaska has the youngest fleet age of any of them. Alaska also has excellent on-time and baggage performance - among the best, if not the best in the industry.
Your first paragraph sums up my experience with Virgin. Hollow flimsy plastic walls, useless seatback displays, uncomfortable seats, tacky college dorm mood lights, horrid customer service, heck most flights don't even offer snacks... I try to avoid them whenever possible.
To be entirely fair, a lot of my grief is based on my experience transitioning my frequent flier miles from virgin to Alaska. They had some sort of fault and I lost 250k miles and Alaska wouldn't do anything to correct the error. They kept saying that the system said I had zero miles even though I was able to show on virgin's site that they showed I had 250k at the time it was extracted for the move. It really soured me on them.
That's a completely fair reason. That happened to me on a lesser scale with American and US Airways (I also lost all of my miles from America West from when I would fly back/forth to visit my father lol thanks airlines).
Totally sucks, especially 250k miles. I'd be weary after that as well.
I try to fly Alaska everywhere too - never had a bad experience, on-time every time.