This is not a post in support of a change in block size or for sidechains, I'm not qualified to take a side in that debate at all lol. What are the consequences of a block size increase and more specifically how it would be harmful for bitcoin's decentralization?
Disclaimer, I can't say that I'm qualified in this matter
But from what I heard, bigger block sizes (like 8-16M) are not good for the resilience of the network since they lead to excessive centralization of mining. Bigger blocks will propagate slowly, so small miners will be at a disadvantage. There are probably other reasons why big blocks are not good. Further, if we want real scalability, Lightning Network is the way to go, not increasing the block size
Regarding pruning the blockchain, I think it is also possible. For example, it might be possible to make a "cold reset", so to speak, i.e. switch to a new and empty blockchain with only initial balances set without the overhead of transactions which led to this state (and we can do so as many times as we need)