I don't really agree that it's a matter of time. There's a pressure, sure, but a stronger counter-pressure is simply having wider product categories and to a lesser extent enough employees to allow for some specialization.

The moment you do web application development, chances are you're using at least two languages if not more. There are some all-JS web companies, there are also some all-Clojure/ClojureScript companies, but neither are very common. Add a database, you might have another language (or not). Add a mobile application, and you're looking at potentially 6 languages just for that product category depending on how you set it up. DevOps? Take your pick, probably at least two used at the typical company that has such specific roles. Survive long enough (many companies don't even last a decade) and maybe make some acquisitions, now you've got acquired product X in some other language -- do you do a rewrite? Another aspect of surviving long enough is the language you picked might have changed a lot. Modern Java, JS, Python, and C++ are all significantly different now than they were not too long ago. Companies with big code bases can't afford a rewrite or mass update, so they have a mix, and developers need to be aware of the conventions of the old versions that may as well be called old languages. Of course some companies will insist on banning the modern styles and idioms even if they begrudgingly update the tooling, for much the same reasons they ban different languages.

Some companies do end up being long-lived and monolanguage though. And where I would agree with you is that individual products will tend to single language policies even if technically they can intermingle (i.e. all the JVM mingling). (Edit: and seeing your other clarifying comment, yeah, top-down control over letting teams make most of their own decisions probably is inevitable with time, this expands far beyond just language choice though.) Sometimes that language is Common Lisp! There are things that can be done to influence what it's likely to be, if it is to be. The "simplest" is probably just having a big enough, complex enough, and important enough product that no one's going to authorize a rewrite for even if the will and expertise to do so appeared. Then the monolanguage will be that language -- whatever a company starts with has a good chance of becoming The Language, but if you're starting with a not so popular language you're in danger of having it changed at some point and will need to protect against that if you care. (This to me explains Reddit's rewrite off of Lisp early on, the code that was written wasn't that big or complex yet, and also explains Facebook's lack of move off PHP and herculean efforts with Hack to compensate. But as companies Reddit seems much closer to the monolanguage endpoint than Facebook -- Facebook simply has way more engineers and a lot more projects going on.)

Another driver is what you highlighted, lack of interest, or indifference, and is probably the main one until you reach a "too big to change" tipping point. You need to have someone who loves the particular stack, or (also?) who hates the alternative stacks, because if you don't someone else will eventually come along with enough ambition and motivation (either from hatred of current stack or love for a new/alternate/more familiar stack) and the stronger passions will win the day with the indifferent employees just going along with it.