Just because you don't see functionality changing doesn't mean it isn't. Lots of new features, just not for the normal users. Not to mention all the backend work to make all this data flow around, be accurate, and be reliable.
And also, there's not just engineering. HR, Accounting, Lawyers, Trust and Safety, it's all there
The question is valid though as to why they've kept growing if the user-facing product hasn't changed. There are some good reasons but lots of bad ones too.
The point I'm trying to make is assume that user-side functionality stays exactly the same - frozen - for say 10 years. Given what you say above, it would be reasonable to expect the company to continue to grow in any case on its current 10-20% compounded annual rate with no end in sight. Is that truly sustainable?
Which users? Internal users? External users? External developers (I know, I know) or academics consuming their APIs? Advertisers?
Starting from the position of what don't I know about this situation, and / or what systems could cause it to be in this seemingly intractable state leaves you open to all sorts of new learnings vs. assuming there aren't intelligent, capable people on the other side.
Twitter is a platform that offers a product. Your attention is the product.
The part of Twitter required to maintain a decent stock of product has not needed to change outwardly, because it hasn't had to. There hasn't been a new version of human in a very long time. But even then, anything that creates more product (drives more engagement) means more revenue opportunity for the 90% of the platform you will never touch.
The parts of the product that theoretically make revenue for Twitter have changed significantly over time. Analytics, ad intake/spend, promotion for influencers/brands/etc.
And also, there's not just engineering. HR, Accounting, Lawyers, Trust and Safety, it's all there