In your scenario, transaction fees must rise because the network is willing to pay to maintain the security budget.

Here's another scenario... transaction fees stay low, even below the security budget, because the ecosystem has evolved to withstand occasional attacks, and participants are not willing to bear the cost of high transaction fees.

This scenario actually describes the Bitcoin Testnet pretty well. Sure, testnet is always getting re-org or 51% attacked. Its inconvenient, unpredictable, and sometimes unusable. But for the most part, transactions still get processed, and txns that have thousands of confirmations are still extremely unlikely to get undone.

The testnet3 lightning network is surprisingly robust: https://mempool.space/testnet/lightning/nodes/rankings/liquidity

Just because BTC has never successfully been attacked like this, does not mean it is so fragile that it will die if an attack ever does succeed.

There are ways for you to guard yourself in an environment where BTC is suffering from attacks:

Owning a LN channel that has been open for +100,000 blocks is probably a good investment in either scenario.