Tablets should have a HDMI/Displayport in so that you can directly use them as displays.
I'd even go one step further: we should have had a standard communications protocol like TCP for all devices. So a display would show up as just another device that we could use to read/write bytes. All devices would have a standard queryable HTTP/HATEOAS self-documenting interface. And HDMI/DisplayPort or USB A/B/C/.../Z would all use the same protocol as gigabit ethernet or Thunderbolt or anything else, so the band…
On the contrary, Microsoft tried really hard with UPnP/PnP-X/DPWS/Rally/Miracast*/etc but nobody was interested.
*BTW any Windows 10+ device can act as a Miracast sink (screen) so you can link Windows laptops/tablets as extra screens without any additional software.