Interesting discussion! Plus it's mostly staying civil despite strong opinions, so pat yourselves on the back.
You're really stuck on this illusion that Disney has been planning singular IP lands for years, yet there is very little evidence to that fact prior to the late 2000s/early 2010s when it has become the new fad of themed entertainment and Disney saw how they screwed up with underestimating Potter, now in pursuit for a 'Potter Swatter'
Also, argue all day that the expansion is a positive in the long term. And I agree. But Bob Iger's whole idea of paying for the Avatar rights was for a bigger attendance boost. I'm not sure he got any bigger an attendance boost than he would've gotten without Avatar. Although apparently they're considering a phase 2, so color me confused. Bob is so out of touch I could never reason with his thinking.
Re: HP and single IP lands.
People often forget Carsland was actually announced before Uni announced Potter. This is not to say Disney clearly didn't know about Potter, nor do I want to say Disney really underestimated Potter and the power of IP based lands. Nor did Universal even know what they had their hands on to be frank. Neither had a clue how well it would do! Or even that Pandora and SW:GE were forced into existence indirectly because of Potter.
All of that is to say Disney still quite independently developed a very successful IP-based land in concert, they just take forever to build things. I don't think Pandora would have happened if Potter failed to exist, I do think Carsland would have still eventually lead to Star Wars. I think more likely the diminutive 1.0 version would have been approved rather than the board demanding something much bigger to compete. But Carsland very much woke Iger up to park synergy, not just Potter.
I wonder how much will capacity go up at DL when SWL opens? Do we know this or have any educated guesses?
I'd say each ride can independently and likely will hold between them a good 8k people. What's important is that 6 - 8k or whatever will be physically held up in queues and out of quite literally any other area in the park - including the walkways of SW:GE itself.
Interestingly, there are actually diminishing returns of "occupied" guests with higher ride capacity. Sure, Pirates can theoretically take on 3k people an hour, but at any given point in time are there actually 3 thousand people in line and on the ride? No, not usually, because lines remain diminutive simply because the capacity is so high and everyone who wants to ride can.
A ride like the battle escape is both shorter and perhaps even half the theoretical capacity. But a two hour line at any given point in time is legitimately holding 3k people, "occupying them". A show like Frozen is different in that the daily capacity is nothing like Pirates, but during showtime a packed theatre is actually occupying a large number of guests for a long period of time. Its occupancy of guest time is reasonable as a result.
Beyond that, the land itself will easily offer a multi-thousand guest free range space. Conceivably, a quarter to third of the parks daily visitors can easily be shoved into that land. If it's overcapacity? Still good, guests are cordoned off into waiting pens to get in.
To conclude my now mini-essay, I earnestly believe Disneyland prime will actually be more pleasant than it is today, unless attendance soars in excess of >3 million. I've been around enough recently opened lands (Pandora, Diagon Alley) that I've found while the new areas are banana's, the park itself is actually super pleasant to visit; simply because so many guests are busily occupied and out of the way in the new thing. That only happens with new areas and not a one-off popular attraction in an old area.
The biggest concerning times may be Fireworks and Fantasmic. SW:GE may need to offer a nighttime draw once the allure of line reduction during showtimes fades.
To answer your original question, 15k seems reasonable. 10k would be my guess for how many people will typically be found in there at any given time. That's a ton of people occupied.
And of course parking and guest arrival to the resort area... THAT's the real bottleneck.