A lot of folks seem to forget that Twitter also has electricity bills they have to pay directly or indirectly. Serving every tweet, along with everything else you see on the internet, does cost money. Usually it’s so small we don’t worry about it but it’s certainly not zero.
The cost of running Twitter could be very small, if they optimized for that.. cost of the engineers is harder to keep down.
But the real problem is probably that they got investors, went IPO and somehow need to produce significant returns.
Maybe Twitter can produce higher returns with a more captured user-base and a less open platform.
It just seems sad, that we're putting walls all over internet.
In any case, I doubt it's cost of operations that causes it. More likely too much investment.