The future is bright.
Since the code is open source, nothing stops you from maintaining it even if the community somehow vanished. The best way to make sure Tiki will be alive & kicking in 5 years is to participate.
Tiki has a much larger development team than most projects. Tiki has (in a very short period) reached a certain maturity and now has a fairly large userbase. Tiki has, in my humble opinion, as much chance as any open source CMS to be there in 5 years.
Tiki is much more than just a wiki. As a full fledged CMS, some people may mostly use the wiki, but others will use other features. This versality increases the chances that the project will survive.
Now, what about things Tiki relies on?
PHP has a very high probability of being around in 5 years. No need to elaborate further here.
Tiki uses a database abstraction layer so we are "future-proof" in this respect. If one database is "gone" or no longer open source in 2 years, just use another.
What if Tiki dies?
I don't think Tiki will "die". However, a project better meeting your needs may come around. All your data is kept cleanly in the database. So if ever you did want to change projects, it remains an option to write a migration script.
Tiki uses PHP (not Perl) and AFAIK, works with any version after 4.1.2 including PHP 5. Apache 2 is supported. The only issue AFAIK, is there may be some modifications to .htaccess files.
Now that's the interesting question! Please see: UserPagemarclaporte at the section "Tiki bloating & featuritis". This is a frequent comment & concern. I personally am not worried.
Tell "management": Nobody ever got fired for picking Tiki ;-)