Absolutely agree.
I don’t know the slot of N900, but the slot of Sony Xperia is more suitable to the concept of Librem 5, because it does not require a pin for ejection and this is good for being able to get rid of the SD and SIM in case of emergency.
It would be nice to be able to hot swap the SD card.
Btw. what is actually the purpose of a SIM tray on the side in the first place? Could it be that companies started to use it when they stopped delivering phones with replaceable battery? If this is the case, then maybe Librem 5 does not really need a side tray.