Sidechains are great in fact. The problem many people have with blockchain is that bitcoin is working worse and worse because it is restricted by blocksize limit. The developers don't want to fix this but instead suggest using the lightning network, which comes from blockstream. So alot of bitcoiners feel like they get extorted by the bitcoin core wallet developers to use the system they create besides bitcoin.
That is the problem many have with it.
I don't think blocksize is the most important limiting factor. A few other factors slowing the network are -
Long block generatiion delays
Verbose transactions
Under-utilisation of blocks
SPV mining
Interference with node synchronisation.
Invalid transactiuons
and a number of other factors that more knowledgeable Bitcoiners have mentioned
Increasing blocksize is just a trivial easily understood attempt to solve a fairly complex problem, without considering the future of Bitcoin imho.