Will the 265 series ever get Stamina?

Realistically, no. Garmin has drawn the line in the sand right now price-wise, and Stamina is currently above that price line (whereas as Training Readiness moved to below that line for the FR265 versus above the price-point of the FR255).

Thank you. This is what I suspected...I guess I will stop being lazy and just switch to Fenix for certain workouts.

For the sake of curiosity, do you know how Garmin determines what price-point a metric belongs in? Newness? Algorithm complexity? The characteristics of the metric iteself (i.e, stamina is mainly an endurance metric, therefore would be more appropriate for advanced/elite watches vs entry-level)?

The price point shifts over time. It's a blend of newness and who it appeals to customer-wise. Meaning, Stamina as a metric likely appeals more to the endurance athlete and likely more technically advanced person, rather than someone going out and doing a 5K. Historically, that meant Garmin's higher end/longer battery life watches. Of course, as time goes on, these previously lower-end watches have far longer battery life than their older siblings, thus, that breaks down a bit.

But generally Garmin launches new metrics on higher end watches, and then each integration will bring it down a level to the next lower price point. There are some exceptions in the 'wellness' category (like the Venu series with ECG).