Recent advancements such as "Segregated Witness" by Pieter Wuille and "Drivechain" by Paul Sztorc showed the power and flexibility enabled by soft forks. But the same technique can also be used to effectively raise the 21M limit, by issuing "soft-fork-currencies" (SFCs) whose protocol rules are the same as those of cryptocurrencies.
A miner majority (i.e. like a softfork) can force changes to the Bitcoin protocol using the method described here:
https://bitcointalk.org/index.php?topic=1296628.0The above proposal was for blocksize limit increases (mildly controversial), but the same idea could in principle be used for other changes that would otherwise require a hardfork.