So, both Bitcoin and Monero would both "break" if they saw sudden massive adoption. Monero would break sooner than Bitcoin, but the margin is not that big a difference. It's something like ~3x or ~4x tx size of Bitcoin. But Monero is far more efficient if you want your transcations private vs making a Bitcoin tx "private". Most of the current blockchain size of Monero is from the past when txs weren't reduced by ~80% of current size.

Bitcoin would be unusable to 99.9% of the world just based on tx fees if that happened, and at a slow rate of adoption this would still happen anyway. While Monero becomes cheaper per tx the more users there are.

So it all just depends. I think rate of adoption VS consumer tech advancement is the biggest factor.

Lightning cannot onboard the world either and comes with many downsides vs on chain. In high fee environments it is not particularly cheap, and keep in mind your channels can always be force closed against your will making you pay high on chain fees. npub169n9eaf0t20j0nefwqlqtnqcpsym22k2nw6e3tevtrrru4et7wrsh5w47v