It may be arbitrary, but increasing blocksize to mitigate network congestion is not a good solution, since any cap you set it to will also be arbitrary, and har

It may be arbitrary, but increasing blocksize to mitigate network congestion is not a good solution, since any cap you set it to will also be arbitrary, and harm decentralisation (block propagation time, node operation costs, etc). And given how valuable it is to have data in a semi-permanent near-immutable distributed ledger, that block space would be filled up pretty quickly, solving nothing. Keeping the limit however has inspired a number of novel innovations to use the space more efficiently, including segwit (effectively increasing blocks to 4MB), and taproot. In short, block size increases are an "easy solution" that don't really solve much.