I think that's a sound appraisal of the situation.
I'm curious to see what Microsoft do at E3 in a week's time. If they announce a "nextbox" that has a budget SKU that is coming in 2019 then it's fair to say Xbox One sales will stall. This will be especially true if their upcoming games are also on PC day and date. It sounds stupid to announce something that early to cannibalize your own sales but don't forget this is the company who announced the Xbox One X about 18 months before release.
One thing is for sure, Microsoft can't wait for the next gen to start. Their most successful console The Xbox 360 was released a full year before the competition, does Microsoft think going early again will achieve the same results?
I don't think we'll see any hardware releases this year from MS. If they have two SKUs differentiated by power level, I doubt they'll stagger the launches. I also don't think they're intent on getting a head start, because having one has never guaranteed success over the competition. In Gen 4, the Genesis had a head start over the SNES, yet it still came in third in Japan and ultimately fell just short of the SNES in the U.S. In Gen 5, the Saturn debuted a few months before the PS1 in the U.S., but its sales tanked thanks to a bungled launch; the PS1 itself debuted a year before the N64 in the U.S. to lackluster sales, trailing behind the N64 up to August 1997, and didn't become the dominant system until after FFVII was released in September that year. In Gen 6, the Dreamcast had a decent launch but declined quickly despite having a considerable head start against the PS2. The PS2 itself had a head start against the Xbox and GameCube, but considering it was riding the momentum of the PS1 I doubt its success had anything to do with its head start. The 360 had a head start over the PS3, but it did not have an outstanding first year and its sales were largely stagnant in the U.S. until the 360 S was released in 2010 and in Europe until it received a deep reduction in price. Finally, the Wii U was first to market for Gen 8, but that clearly didn't do anything for it.
Seeing as there is no conclusive evidence that having a head start helps with market share, I doubt MS will try to get the jump on Sony for Gen 9. I think Nov. 2020 is almost guaranteed for both the Xbox 4 and PS5.
Even an announcement is unlikely to have an effect. Historical sales data shows that a system's post-peak sales decline does not start to accelerate until after its replacement is released, so I still expect ~5M for this year and then ~3M for next year.