Both MS and Sony have every interest in launching their 9th gen not earlier than Q4 2019, because the game they can play is different from Ninty one, so they need to play their own game and offer a big performance jump, and possibly VR support and other features, at a decent price.
And if they decided to play Ninty game and launch hybrids, they'd need even more time, even if they started developing them as soon as Ninty revealed enough infos on the Switch, their machines wouldn't be ready before Q4 2020, but 2021 would be better to give 3rd parties enough time to offer enough games at launch (MS and Sony need more 3rd party support than Ninty to succeed).
So yes, MS releasing XBX and not rushing XBTwo is good for Sony, but it's the best choice for MS itself too.
About sales, it will have roughly the same relative success as PS4 Pro compared to their respective base models, giving XBOne roughly the same percent boost PS4 Pro gave PS4, so it won't ever give XBOne the victory, but it will surely better its total sales and make PS4 domination a little less embarrassing.
Against it there is its high price, in its favour its higher performances, so its appeal on techno enthusiasts, and the fact that Sony underestimated PS4 Pro potential, producing for a while fewer units than it could sell, this can help MS to estimate better the number of units it needs at launch to fully exploit its potential.
Stwike him, Centuwion. Stwike him vewy wuffly! (Pontius Pilate, "Life of Brian")
A fart without stink is like a sky without stars.
TGS, Third Grade Shooter: brand new genre invented by Kevin Butler exclusively for Natal WiiToo Kinect. PEW! PEW-PEW-PEW!