Because the next gen games will expect faster data transfer than an external HDD (or even an external SATA-SSD) can guarantee.

I'm sure that you can move the next gen games to the external drive instead of deinstalling/deleting them. When you want to play the game again, you just move or copy them back to the faster storage.

That is basically what Sony and MS said. External HDD you can use to last gen game directly, for current gen game it needs to be on the internal SDD or an equivalent external. If you have the next gen game on a HDD it needs to be transfered to a compatible SSD before run.

Its a shame the Series S leaked the way it did but at the same time it was the worse kept secret for any console i history. And I think the viral thing worked it it's favour.

It was trending for quite a while on most media outlets.

I get that, what I was implying is they should have bloody thought about this and made it work with their new custom SSDs as externals.

Saying that, Barozi pointed out that there indeed (at least on the XBOX) is an expansion slot for their propitiatory SSDs. This is great news and glad they actually put some thought into it. Hopefully the PS5 has done something similar.   

From what we know Sony will have certified (but don`t need to be) expansion driver for internally, but externally will be USB 3.1 or perhaps 3.2 still not sure if even a capable SSD on the external would run nextgen game.

The HDD news isn't new, they revealed this months ago. Last gen and older games you can store and play from any external drive but for XSS/X games they have to be on the SSD or an official external SSD. AFAIK the PS5 is the same way, though they will have third party drives. Also as others have said, this was supposed to be a briefing for media and retailers, not the general public, so of course it's a bit dry.

Yep and that makes me happy that there is some way for next gen games to be stored (even if it costs me more for their certified expansion drives).

Main reason I wanted this ability was tired of all the day 1 patches and addons, that you have deleted only to years later not being able to play it.

Being able to play last gen games and store them on a regular external is also an awesome addition. Finally I never have to delete stuff, just buy multiple drive. Hell even back up drives if I feel like it. 

I imagine games optimized for Series X actually require significant work, essentially be a port to really utilize the new hardware.

Unoptimized will still have significant upgrades like fast loading, dynamic resolutions staying at max, performance actually locking to 30 or 60 fps, etc. I recall MS also demonstrated Gears Ultimate was running at native 4K even though the game has no 4K support on Xbox One. Upgrades like that might just be at some emulation level but still not an optimized game.

I'm coming from One S so even if a lot of the games I play currently get bumped up to the Xbox One X versions, I'll be pretty wrapped. Apex Legends for me, for an example is 720-1080 dynamic 60fps.

Kinda can't wait to see it in 4k tbh.

Leaks build up hype. Reveals do not... or not as much.