The question of short term incentive should be considered separately from the question of suitability for long term storage of value.
For the short term incentive, it is expected to converge towards an equilibrium. When the expected return of STX for one BTC is lower than the expected exchange rate, miners will reduce their investment, leading to a higher return of STX per BTC. And vice versa.
Regarding long term (e.g., for 12 years) storage of value, it’s probably too early to determine. No one does even know for sure whether BTC will survive when there is no mining reward anymore. But as long as it takes considerably less time to convert the STX on an exchange, this should not be of concern since the PoX mining reward can be converted to any asset you consider suitable for long term storage of value.
In the meantime, however, it should not be ignored that the STX that are held can be used to receive additional BTC through stacking. Considering that the locking time required for stacking is comparatively low with respect to the time until there will be a difference in the reward strategy of STX and BTC, this can be relevant for assessing the incentive.
In order to depend on this, it’s of course important that the STX blockchain does actually behave as advertised. So, I think this recent observation is also relevant.