- They enormously extend their offering to support development of arbitrary server-side code.
- They figure out how to do this in a way that doesn't introduce inescapable lock-in for application developers.
- They figure out how to differentiate this offering from what similar server-side deployment/platform services already provide (Heroku, EC2, AppEngine, whatever).
That's not to say UA or Parse won't ever see a big exit (in this market, who knows). Rather, I seriously doubt -- in either case -- that the economics are there for more than a profitable "lifestyle business".
(no negativity implied -- I favor lifestyle businesses, but VCs don't).