I guess you are right in the canabilization. I would definitively sell my 28-70mm F2 if I bought the 35-135mm F2, even though the 28-70 F2 is by far my most used lens outside of sports (100-300 F2.8) and wildlife (100-500mm probably, but EF 400mm f2.8 iii, EF 600mm f4 ii, RF 200-800 and 100-300mm).
I do have some hope, though.
The RF 28-70mm F2 was released 7 years ago. There have been plenty of mentions of a MK II, especially after Sony came out with their much lighter version last year (918g vs 1430g). The RF 28-70mm F2 was a trailblazer, a halo lens, the kind of lens that makes you want to switch systesm. A 70-135mm F2 lens would be really underwhelming in the face of Sony's 50-150mm. Canon releaseing their own 50-150mm F2 would be great, but not exactly a standout lens. A 35-135mm F2, on the other hand, would be a one of a kind new lens. While 35mm isn't exactly wide enough to replace a standard zoom, 28mm barely is, it is wide enough to make it quite usable in a lot of settings! Yes, the 15-35mm f2.8 could complement it very well. Actually, even the RF 16-28mm F2.8 could fill the wider end.
Well, the current Sony 50-150mm F2 (1340g) is ligher than the Canon RF 28-70mm F2 (1430g). So it is possible to have a 50-150mm with reasonable weight, that is, if you can manage the Canon RF 28-70mm F2, a 50-150mm F2 should be manageable.
it is priced at $4k vs the Canon 28-70mm F2's $3.3k, but the Sony is a newer lens. So the cost isn't that different.
I agree that all things equal, lighter is better

. But, if we fix cost and IQ, then, for me:
Good trade-off:
50-150mm F2 -> 50-135mm F2 that is lighter/smaller
50-135mm F2 -> 35-135mm F2 that is heavier/larger
In other words, I'd trade-off focal length range on the long end for size/weight, but I'd happily deal with larger weight/size to get move focal length range on the wide side.
If 1kg is already heavy and at the max for you, then perhaps an RF 28-70mm MK II at ~900g and an RF 70-135mm F2 at ~900g would be the ideal. I'd go for a 1500g RF 35-135mm F2 in a heartbeat!