There is no fixed block size issue because by most people's estimations, something like the relay network is required for over 500k-ish blocks. Because there is currently no alternative to the relay network that isn't opt-in, this just makes it a cartel creation type of tool to exclude various adversaries from interacting with the network at all.
Thin blocks are a reasonable ad-hoc alternative to the relay network. It is a bit less efficient but only by a small factor (and probably not even that if further optimized). Works just fine for Monero too.
Monero also doesn't suffer from many of the script-related scaling bottlenecks in Bitcoin (or worse Ethereum), because it isn't trying to programmable money or a world computer and is focused on doing one thing and doing it well (A pays B). That said, Monero will probably have some sort of programmability at some point,
when it is understood how to add this programability as an optional enhancement over the optimized base function in a scalable and suitably-private manner. (The recent update on Ring Multisig being one small step in that direction.)