I think it will, but probably not by much. Assuming the XBO is down 30% YoY, that should put it at no more than 5M for this year, which would put it at ~46M by the end of the year. Assuming a 50% decline next year (because of the launch of the Xbox 4), then that will put it at ~49M, right on par the SNES. Assuming another 50% drop-off in 2021 and again in 2022 with a 2023 discontinuation, then it will likely end up at around 51-52M lifetime.
Looking at it regionally, according to NPD the SNES sold 20M units while the XBO is sitting at 25.1M as of April of this year. VGC has European sales of the SNES at 8.15M while the XBO is currently at 11.52M. Meanwhile, in Japan the SNES sold over 17M units while the XBO has barely broken the 100k mark. So, the west will be entirely responsible for the XBO beating the SNES globally, assuming it does so at all.
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?