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.
The long block generation delay is a real setting from the foundation of bitcoin. The blocksize limit is not. It was set arbitrarely to protect against spam. Which it never did until the blocks became full and it became affordable to spam.
It wouldn't be so bad to wait 10 minutes for a confirmation IF you could predict the outcome somehow. If you can't know if your transaction will go through at all and you have to wait maybe alot of hours then this is way way worse.
I googled what you might mean with verbose transactions but I did not see what you might mean.
Under-utilisation was never a problem when blocks only were full a couple of percent. Why should it be when blocks were full a third again? Or do you mean the zero transaction blocks? Yes, that is surely not helpful. There should not really be a an advantage anymore doing that. There are ways to propagate full blocks in a matter of seconds already. Including >50% acceptance.
But this behaviour can't be stopped except with a hard fork. Which surely won't happen.
SPV Mining should have become a smaller problem now. Those doing it promised to verify the transactions of the previous block though while they mine on that block already. Verifying takes time so it is understandable. It will happen again probably but it should not be a big problem as long as the company mining that blocks doesn't have a huge share in total hashrate.
Interference with node sync? You mean ddosing?
Invalid transactions look like a problem to me too when spammed. Though it should not be a huge one except delaying nodes with the calculation. I did not hear of an actuall attack through this.
The future of bitcoin inevitably needs a raise in the blocksize limit. Otherwise you will be stuck with what we have. Segwit can only do his part until there is no way to improve. And not using bitcoins but sidechains instead... well, that sounds like giving up on solving problems. Surely I will not have my bitcoins being bound in a payment channel... and so on...