What if a bunch of people joined and decided that Perl is not relevant and we should start discussing python/ruby/js ?

Then an appropriate thread would be started, and either a new site or section would be opened. All by suggestion and vote. Again, that is the part of the beauty of this site.

Wouldn't happen.

Which part? :p

The part where a bunch of relative newcomers would be able to sway the site entirely away from Perl.

One important aspect of our bureaucratic momentum is the fact that one must be at least a Friar to have moderation power. Then, of course, one can't be a janitor or other power user without the action of the gods, who tend toward the conservative on such matters. So for your scenario to have any realistic chance of happening, it would have to be a massive, highly coordinated campaign, over a very long period of time. So, yeah, if the Chinese government really wanted to undermine PerlMonks, they could possibly do it. :-)

... either a new site or section would be opened.

There's a big difference between a site and a section. If many people wanted, say, a "JavaJunkies", they'd just go and do it. No restraints; no votes necessary. If they thought there should be a new section, well, they're likely to be disappointed (not that this policy is absolutely immutable for all time, however).

All by suggestion and vote.

I'm curious how you expect we could run a vote with sufficient participation such that the results would be considered anything close to a mandate.

I know you didn't say specifically that we could use the site's existing polls feature, but I'll note that, out of the 425 polls to date, the feature was almost never used to solicit real input from the monks, and not since 2001 or so. (Should Perl Gurus receive mega XP?)

Relevant discussion: Polls on PM issues

The real problem, with polls or any other technique we have or might come up with, is in getting sufficiently broad-based input from the membership. If only a few people weigh in, why should we consider that compelling? And, yes, you might argue that "Maybe only the active members' opinions matter anyway," but how do we know who's really active? If vkon, say -- who as of today hasn't logged in in several weeks -- misses the discussion and poll, then we're misunderestimating the active membership, and the results lose trustworthiness.

Elsewhere, tye noted another problem with using polls: "Trying to build a democracy out of a poll where only a single person gets to compose the question to be voted upon" would be stupid.

Lastly, I think it's worth noting -- though it may seem anathema to some -- that not everyone's opinion is equally valid. I'd be far more inclined to trust a consensus of dozens of long-time members than of hundreds of newbs. Not because seniority is inherently more worthy -- I hate classism as much as anyone -- but because they have a greater familiarity and understanding of how this site and this community work.