Stretch seating is essentially the same as Economy Plus on United. The extra legroom is a definite plus. Other than that, it's a conventional seat with Economy service.

As a 1k myself, it would be very difficult for me to lose that level of elite status. Besides the points you mentioned (up-grades, etc.) there are also a bunch of other benefits to consider. Some that come to mind include:

1) Priority on the stand-by list regardless of when you checked-in.
2) You're accomodated much, much better during irregular ops than regular passengers are.
3) The 1k desk will open up award inventory for you when seats aren't available to the general public or lower tier elites.
4) Bonus miles based on elite level and fare class.
4) United has hubs at LAX and SFO and thus a much larger West Coast network than Frontier. This can help in cases where non-stop flights to DEN are sold-out/cancelled or if you need to make multiple stops.
5) United is part of the Star Alliance and as a result had a very robust global route network. While most of your travel is domestic, this can help a great deal when flying overseas for vacation, securing up-grades, etc. Frontier has nothing.
6) Frontier will not provide a status match to their top elite level (unlike other airlines).

While Frontier is a good airline, they do have some pretty significant limitations as well.