There is no decision making mechanism in the current bitcoin core devs group, thus the chaos
In fact bitcoin is centralized around its protocol, the protocol is very centralized, there is no freedom in selecting different parameters of the protocol, that's the only thing that all the bitcoiners agree upon. So the protocol should be highly centralized around those basic promises like limited total coin supply, shrinking block reward and block reward incentivized mining
But block size limit clearly showed that even a small disagree on one of the parameters of the protocol can make it split, so it is better to let the rules written into stone and no one would be able to change it in future (So that we don't run into these political fight again and again in future)
Satoshi said the block size limit can be lifted when needed, but even raise it to 20MB will not be a permanent solution, a permanent solution will be either totally removing the hard cap (soft cap can still be set by miners) or set the cap by a dynamic formula. A genius formula will be the real solution but that requires huge wisdom, maybe Satoshi will throw in a formula some day
That is the holygrail of bitcoin.
I agree that the obvious solution is a sliding scale for the block size. You think it would be easy because they already do this with the difficulty. However I know little of math and I am sure that it is more challenging than it seems to the layperson.