It's 20% off 512gb Steam Deck, the 64gb model is 10% off and the 256gb model is 15% off.

You can have a swappable battery and water resistance. Samsung galaxy s5 had a removable plastic cover and IP67 water resistance, so if the manufacturers tried they could easily have them both.
Also, if you do any repairs on a new phone you lose water resistance (unless you replace the silicone), while on the s5 you could just replace the battery and would just need to press the cover back on. After the repair the phone would still be water resistant, unlike the new phones with glass backs.

Maybe there is no option in the settings (yet, maybe they will add it later), but you can still start steam minimized to system tray:
https://help.steampowered.com/en/faqs/view/0188-6BB7-D467-08E1