Bergoglio has admitted that the Motu groups have served as a "gateway drug" or "halfway house" to Catholic Traditionalism. When the faithful get exposed to the True Catholic Mass, it tends to make their doctrine orthodox as well. So he wants to stop the ONGOING bleeding. Sure, in the short term, the numbers of SSPX trads will swell, but it'll be a burst effect, rather than a longer-term process of drawing people out of the Conciliar swamp.
If I had to guess, when Bergoglio shuts down the Mass, maybe 25% of the Motu attendees might go to SSPX, tops. There's a reason that they're at the Motu rather than with SSPX already, some of it being the nicer/bigger churches, nice choirs, shorter drive, etc. ... but the majority are there instead of SSPX because they don't want to be "schismatics". Of the remaining 75%, perhaps half would gravitate toward Eastern Rite, and half would go back to the NOM. So he's willing to cut loose that 25% to keep the remaining 75% from gradually also tending in that direction (and the future generations of that 75%). That's his gamble.