They gave up over a billion dollars in revenue in order to subsidize Xbox One sales. It's very likely that won't happen again. Not only is it expensive, it'd be much less effective in the current situation. A ~600k unit swing wouldn't halve the gap, they'd need an effect twice that strong to do so.
These statements are misleading. First, by averaging the consoles together you're producing a false comparison. Microsoft and Sony do not average 4.9m units per year; Microsoft averages 4.7m and Sony averages 5.2m. More importantly, that second sentence greatly undersells the scale of the disaster it would take for Microsoft to catch Sony in the US so rapidly. Xbox One sells about 2.4m per Nov/Dec, so PS4 would have to literally sell zero consoles for "one bad Christmas" to eliminate the gap. This is an edge-case fantasy, not a living possibility.
Your facts are sound, but your takeaway less so. Yes, the holidays are half the story each year. But concluding they're the only season that matters is refuted by the actual sales pattern. Xbox One is very successful versus PS4 in the holidays...but they're losing the US anyway. Each year, PS4 builds up a lead in the other ten months, and each year Xbox's bite back at Christmas is insufficient to balance it. The unit gap is higher than it's ever been. The percentage gap has been climbing for six months. That is, for the last half-year PS4 has not only been beating Xbox One, it's been beating it by a higher margin than before. So far, 2017 is the worst year Xbox has had. It's the best year PS4 has had.
And this comparison doesn't just hold with PS4. The pattern of concentrating so heavily in the holidays also differs from the path Microsoft themselves took last generation. Xbox One is above Xbox 360 in the US launch-aligned, and likely will remain so deep into 2018. But outside the holidays month-by-month performance does not look impressive against their prior machine. It seems not so much an intentional strategy, as the best of a bad job. Even the holiday advantage has become less effective over time.