I'd prefer a bit more reach, even knowing there will be a weight/size increase. 70-200/2 would be ideal, 70-150 at a minimum (either could start at 80mm). Personally, I would not switch but rather bring the 28-70/2 and the longer f/2 zoom for low-light events, and the 24-105/2.8 and 100-300/2.8 (or 70-200/2.8) for better lit or outdoor events.
You can't really say the mount is closed anymore, since Sigma has
announced 6 RF mount lenses. But we don't know when/if Canon will allow FF 3rd party lenses. Still, personally I'd take the 28-70/2 over the 28-45/1.8. The much broader focal range makes it far more useful than 1/3-stop of aperture, for me. The limited range of 28-45mm is functionally a prime lens, so if I needed that I'd just buy the 35/1.4 and take one step back or forward as needed.
Yeah I get you, nothing to argue really, these are just minor differences and personal preferences. It's always a tough choice and a compromise at some point.
I used to carry my 100-400 and 100-500 for events when I did not have a 70-200 and was surprised to see I actually took successful shots at 300mm and above. But it's definitely not something I need. I think 150-200mm is the max on the long end on FF sensor but honestly, if I had a 24-120/2, I'd probably go single lens setup. (I have a friend, for events he has switched to single-lens, using Nikon's 24-120/4. And he is a pro, making a living of photography for 30y now.) Events can be very different too. For a 1h protocol I don't care about size and weight. For a whole-day event, maybe including lot of walking as well, I do. For an outdoor event, 2.8-4 is absolutely fine. For indoors, 1.8-2 can often be essential.
For me, zoom lenses are about not needing to change lens. I really don't need all those focal lengths. I mostly shoot around 35mm, 85mm, 135mm. I could do with 3 prime lenses (actually this is what I do now temporarily). So if it saves weight and size for me, I'm fine with 24-40 / 24-50 and 80-135 / 80-150 / 100-200. The 100-300/2.8 is definitely something waaay too big for an event.
I have been using the 28-70/2 it's a great lens, but even that is a bit too heavy for a whole-day event and it's not just heavy but also nose-heavy which makes it considerably more uncomfortable than a usual 24-70/2.8.
Sidenote, I wish there was a lightweight 100-300/4 for travel. The 70-200/4 is really great and super cute, but the reach is just not enough and I really don't care about the 70-100-120mm part. Now I use the 100-500 but it is still too heavy on a backpacker trip. 120-300/4? 120-300/4-5.6? 150-320/5.6? Something that is max 850-900g.
As for Sigma...meh. It's somewhat a good news but I'm sure Canon is demanding a license fee so lenses will be more expensive. In the meantime, I only care about FF Art lenses from Sigma and the fact that all recent pretty great and serious lenses lack the RF edition and there's no sign of such additions in the foreseeable future is very worrisome.