That's definitely a valid objection and I have absolutely no idea what could be done about that. But as it is the bug that takes down one machine today will take down other machines tomorrow and yet other ones five years from now.
So for entirely new bugs your objection stands, they could (and likely would) be disastrous. Even today a 'zero day' exploit for a major platform can be dealt with though, I don't see why we would not be able to deal with such exploits in a scenario where there is only one implementation of something, it's not as if right now we use the other implementations to keep things running, it's mostly a matter of impact all at once rather than spread out over time and infinitely repeating.
>Even today a 'zero day' exploit for a major platform can be dealt with though
But they can only be dealt with once they're discovered by people with an incentive to fix it. The NSA says they weren't using Heartbleed[1], but I can't think of a single reason they wouldn't lie about it. In any case, that was a massive security flaw that could have conceivably been exploited for more than a decade. If it affects 30 percent of systems instead of 90 percent, that seems like worthwhile hedging of bets.
Afterthought: If we really only had one implementation of something all our energy could go into making that one thing really good, which might even push the quality beyond the point where zero days would be an issue.
[1] http://evolution.berkeley.edu/evolibrary/article/agriculture...