Regulatory compliance is the one I suspect would be the hard one when it comes to switching a company like Uber to maintenance mode with greatly reduced developer staff on their apps and backend code. They would have to deal with regulations changing in areas they are already in, and in area they expanded to might have regulations that aren't just a trivial variation of regulations they have already figured out how to handle elsewhere.
For dealing with growth, there are two kinds of growth to deal with. One is growth within a given metro area that they already serve. The other is growth into new metro areas.
For things like ride scheduling, driver scheduling, routing, and similar they only have to grow their software to handle that in the largest single metro they serve. Multiple metro areas could then be handled by running separate copies of that software in each metro area. That would require an IT staff that would grow as they expanded into different metro areas, but wouldn't increase the number of developers needed to maintain the app and service code.
It would be the things that are shared between multiple metro areas that might need a growing developer staff even in maintenance mode. That would be things like the login system and payment processing. But note that these are things that a bazillion other online companies have to deal with too. It's well known how to do them and there are a ton of libraries and frameworks and services to support that. These are things you get working and then rarely have to touch.
It's the stuff in the first category, dealing with the details of what Uber has to do that isn't something every other online site with a large nationwide user base has to deal with, where the hard problems for such a business lie.
As far as security goes I think the number of developers needed to deal with security goes up much slower than the number of users. It is mostly a function of the amount of code you have, not how many people use it.
So, aside from dealing with regulators, I think a company like Uber could if it wanted go into maintenance mode and greatly cut the number of developers (although not down to the 15 voz_ used in his comment), once they have gotten the parts unique to their kind of business to the point that they can handle the largest single metro area they are in or want to be in, and they have a good enough debugged enough feature set that there isn't room for someone else to come up with a better enough feature set to matter.
This isn't specific to Uber or other taxi-like services. It is that way for nearly any nationwide business where the actual goods or services provided are essentially local.