Spirit isn't blocking middle seats - I flew on them a few weeks ago and they definitely had middle seats booked up. That said, I thought they handled the delay I had better than American or JetBlue - I had a two hour delay on Spirit and we were quickly informed of the delay (including how long it was) and we were proactively offered $50 vouchers for a future flight. JetBlue had a 3-hour rolling delay, and given how quickly they were boarding other planes (and wound up boarding ours) I did not feel comfortable leaving the general gate area beyond very quick bathroom breaks - even walking a few gates down for 10-15 minutes seemed too risky. No compensation was offered, even after filing a complaint online. On AA we boarded and then there was a two-hour mechanical delay on-board (in seats not a whole lot more comfortable than Spirit!) and we missed our connection, and they wouldn't rebook us on another airline and no compensation was offered there either, despite getting home 7 hours later than scheduled and having additional connections - we weren't even given a meal voucher!
Which brings me to this:
I think they'll do a fine job keeping their customer base. Having additional frequencies that are easily able to be rebooked on will also help when there are cancellations, and given that most legacy airlines seem to care even less about most of their customers than the ULCCs, I think just as many people, if not more, will be pushed away from the legacies and willing to try ULCCs (since it's "can't be any worse" than their last American or United flight) as those pushed away from the ULCCs into full-service carriers. The ULCCs also have a huge advantage lately by focusing on leisure travel - that's picking up much faster than business travel, and leisure travelers are far more price-sensitive and somewhat less "comfort/frills"-sensitive, particularly in terms of premium cabins or nicer seats. I think that'll bode well for the merged carrier and, assuming the merger gets approved, unless they massively screw things up they have a long runway for growth available to them.