This argument applies to both XLM and XRP, and is often misunderstood.

On Stellar, the base fee is ~0.00001 XLM.
On XRPL, the base fee is ~0.00001 XRP (burned).
If the token price rises, the cost in fiat currency increases slightly, but remains microscopic: even with XLM or XRP at €5–10, we're talking fractions of a cent per transaction.
Furthermore, both networks are designed for this scenario:
Fees are not percentages
Parameters can be adjusted
The purpose of the fee is anti-spam, not revenue
Saying "if XLM rises, the network becomes unusable" is therefore misleading. A higher price simply means that fewer tokens are needed to perform the same operation.
It's the same concept as satoshis for Bitcoin or wei for Ethereum.
In short:
High price ≠ expensive network, for XLM as for XRP.