Grading will feel different on projector compared to LCD monitor regrades if it's Rec.709 or P3, so this is not a valid argument. Projectors have simply different look due to screen size and overall technology differences.
There is nothing amazing or crazy about P3. If you have P3 capable monitor you may swell use it, specially when these days even consumer TVs are getting there. What are you losing?
As you said- for big movies grading is done to the projectors, not LCD/OLED reference monitors. For mid range projects- you have to choose. I agree that having projector for at last validation is good if you know project is going to be projected in cinemas.
Then you have enthusiast with their projects and I don't see a reason why they can't grade to P3 even if their work is going to be seen on consumer TVs. Quite often they have high-end TVs, so they may swell do it for themselves.
How long are you going to be capped by very old Rec.709? It's not old days when you had 1 grading monitor (costing fortune) capable of actually displaying P3. Today this is getting way more affordable and I don't see a reason why people can't use it. Maybe big audios wants you to think this is something out of this world as they slowly loosing any advantage to possibilities of knowledgeable guy at home
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I'm not saying that everyone should jump and use P3 for everything, but if you have good sources and know that your project is going to be shown with display technology capable of P3 then why not to use it (if you have P3 capable monitor).
Do you really think that "old" Dolby is much better than this new Eizo? Yes, it may display 100% of P3, but this is not a deal breaker when it comes to final look on the screen.