Side chains and payment channels (or payment hubs) are methods to improve the transaction density of the blockchain. That means you can get more economic transactions from a single on-chain transaction. They are great ideas but they still require at least infrequent access to the primary chain.
Maximum supported users based on transaction frequency.
Assumptions: 1MB block, 821 bytes per txn
Throughput: 2.03 tps, 64,000,000 transactions annually
Total # Transactions per Transaction
direct users user annually Frequency
<8,000 8760 Once an hour
178,000 365 Once a day
500,000 128 A few (2.4) times a week
1,200,000 52 Once a week
2,600,000 24 Twice a month
5,300,000 12 Once a month
16,000,000 4 Once a quarter
64,000,000 1 Once a year
200,000,000 0.3 Less than once every few years
1,000,000,000 0.06 Less than once a decade
As you can see even with a low transaction frequency the network can't support more than a token number of users.
When someone advocates a permanent cap of 1MB what they are saying is "I think Bitcoin can live up to its full potential even if it is never used by more than a couple million users making less than one transaction per month.So yes using sidechains, cross chain atomic transactions, and payment channels are trustless methods to reduce transaction frequency but they are multipliers. One could look at the primary chain like a savings account and the alternative as a credit/debit card. I may make 300+ transactions on my debit card each month and only two involving my saving account. The good news is 2 is less than 300 but it still isn't zero.
If alternatives reduce on chain tx frequency down to even just once a quarter or once a year you are still talking about capping the network to tens of millions of users. Does Bitcoin need 16GB blocks? Probably not. It is one option but it isn't necessary. There are other ways to go about reaching scalability but you aren't getting there from 1MB blocks on the primary chain.