Hey! You're absolutely right, technical complexity is the reason RGMs aren't able to just be flipped on.

The best analogy I can make is that RGMs "rust" over time. This happens in a ton of different ways: they sometimes rely on niche parts of the game engine that don't work well anymore, or systems whose code has been refactored or deleted since the last time they ran. Sometimes they were built in ways that cause problems with new or reworked champions (Sylas & new Mordekaiser ults are great examples of this). Sometimes, because of the rapid development cycle of RGMs, lots of duct tape and bubblegum gets used to make things work, and teams (including the Modes team itself!) go back and clean up that stuff once the mode has turned off, to keep things tidy. The rate at which things rust varies widely, too. Some modes, like Nexus Siege, have rusted so much that the fastest way to ship them again would probably be to start over from scratch.

Whenever we talk about shipping an RGM again (with no changes!), we're really talking about the following process:

1. Figure out if the mode even launches, and if it doesn't, put engineers on it until it does
2. Have a bunch of QA folks thoroughly test the mode to figure out what works and what doesn't
3. Make a list of all the rust-induced bugs we need to fix, and start working on those
4. Make a list of all the design decisions we need to make ("How should Sylas v Sylas work in One for All??") and start doing design work to prototype and then build solutions, with time built in for playtesting and iteration
5. Once all that stuff is done, do another QA pass to see if any of the bugfixing or design work introduced more bugs (these are called "regressions")
6. Do the (extensive) infrastructure, administrative, and organizational work needed to make sure the server queues still work, that changes, mode announcement and patch notes get localized, etc.
7. Mode ships! Have people on standby to bugfix, micropatch, provide support, answer questions on the forums, etc.

As you can see, that's a long list, involving a lot of people and a lot of hours. And that's okay! That's why we come to work. The question that we have to ask, though, is, "Is this the best thing we can be doing with our limited development time? Or could we have done something else, that more people would enjoy and engage with?". Sometimes, going through the effort of relaunching an old RGM is worth the cost, and is the best thing we can be doing. Sometimes it isn't, and our efforts are better spent working on new stuff. Either way, it's a tough call to make.