If this is out in 2020, which seems will be the case, then it’s good until 2023, maybe late in the year. So it should be the last of its line. If Canon can’t get a true pro mirrorless out before then, it will be a real problem. If they do, it likely won’t be this year. Again, maybe in 2020. Then, it will be elegable for an update in 2023. That would mean the second gen for that at the same time the life of the 1D might be ready to expire.
The timing will be just right. Announce the discontinuation of the 1D, and the introduction of the second gen mirrorless Pro model at the same time.
I imagine that Pro glass for the R mount will be in excellent supply by then, going by what’s already announced for this year. The very longest lenses can linger for a couple more years beyond that.