Give it time. The number of consumers burned by an acquihire is still relatively low, but it's growing. I've certainly heard people say things like, "sure that TV series sounds great but I don't want to get into it until I know they're not going to cancel it," having been burned by such in the past; if the current model of acquisition continues I think it's only a matter of time before it becomes a matter of common wisdom to avoid getting too hooked on a app, because if it's any good its developers will just get bought out and the app shelved.
As for "never ever", I will say that this issue is currently fuelling my own reluctance to even bother getting a smartphone at all, because I'm working from the assumption that any app I find particularly useful (other than the browser itself) will just go away. So, that's one.
On a different note, I do have a smartphone. But I hesitate as hell to buy apps on my Android (no paid app here) for the very fear that if I suddenly jump to another platform I'll lose everything. Of course, I'm quite new to this "new" smartphone generation.
My previous smartphone was a S60 Symbian over 6 years ago.
Dwarfed by the numbers burned by pivots or outright failure though.
No the sky isn't falling :)
If you're software gets used daily (think POS) or touches important data (think CRM, help desk), people absolutely care if you'll be around some day.