Which is?
Meanwhile some people are expecting that somehow a massive amount of nodes (thanks to the artificially kept small blockchain) will be run on a network that nobody can really use but a few niche areas.
Basically, who will care running a node at all?
Oh... that... well..it would be a legitimate point if it made any sense at all..unfortunately it doesn't.
Without nodes there's no Bitcoin and without Bitcoin there is no payment channels, sidechains, off-chain services, nothing.
So basically by running a node you are not only supporting Bitcoin but all of its ecosystem. Imagine when nodes get commoditized and every one can set easily set up one and maintain it a little to no cost, the ultimate decentralization!
Of course that can't happen if you bloat the blockchain so much that you'd need specialized hardware/datacenters to run it.
Look I'm not here to argue that decentralization + having a maximum of full nodes and security is not important. It is and I do care about that a lot just like you do.
But how does that solve any problem about bloating? The bloat will just go elsewhere btw and there will still need people to run sidechain nodes with massive bloat.
Another thing, how do you expect to keep most of the miners on the main chain if most transaction volume happen on sidechains? How do you know if a sidechain will, at some point, become much more profitable to mine than the main chain? You don't, just like myself but risk is very real because big transaction volume = big potential transaction fees. You should think about the consequences of this very probable scenario.
I'm not sure what is this sidechain nodes you refer to... Bloat is dealt with by layers on top with more efficient ways to deal with transactions, see Lightning as an example. The important thing is not to externalize the cost of this bloat to the Bitcoin nodes which are not rewarded for their work.