Equally important to bitcoin is "be your own bank" and that you have completely sovereignty over your own wealth. This ceases to the case if a small group of developers decide to start siphoning off some of your coins against your will.
Which one is more important remains the question, which feature is more valuable is also a matter of question, is it the 21M cap, or the free storage of coins, hard to tell which one to sacrifice when the time comes.
The same argument could be made when developers start injecting more coins that what you thought possible, I can reserve every argument against the two different methods, they both are "bad results" we just have to debate which is worse.
Why is that very likely? The majority of blocks which are currently being mined already take part in merged mining via the likes of RSK.
Work as in (work to solve the problem of mining incentives) i am aware of merged mining, got a dozen of those useless coins, all fees extracted by all the different merged coins throughout history are not close enough to being worth half the block reward, they are essentially useless to bitcoin, and I don't see how that would change.