If U2F is not cross-compatible, then there is a strong argument to made to always have a second device as a back up stored somewhere secure in the event of loss or failure of your first device.
This is a very correct thought. And in fact, I was fighting back in 2015 to buy myself a spare Trezor. But for some reason i didn't. And he acted rather stupidly. Now I will have to wait another week and a half until the new Trezor is delivered to me. All these weeks and a half, I will not have access to either the mail or the stock exchange. In conditions when there is now a strong growth in bitcoin, it was rather stupid to be left without a spare Trezor. But apparently this will be a lesson for me for the future.
This is where HW security keys may come in handy. Two or even three instances of such devices would be more cost effective that the same quantity of Trezors. Besides having less electronic components ( no display and relating biasing circuits) HW security key is more reliable and will last a longer. I have created a dedicated thread for them.