It will absolutely 100% sell better than MKX.
Let's get into it
So the main factor is the current gen market is MASSIVELY bigger than it was, the potential player base is now huge, significantly better than when MKX launched.
Injustice 2 gained alot of fans and that has alot of overlap with Mortal Kombat now which will also bring fans of that franchise in as well as MK diehards.
Graphics are even better than ever before, like it or hate it graphics are a games main selling point.
Larger DLC plans/continued support. MK will almost certainly get at least two season passes. That's continuous marketing and public concisousness for at the very least a year. By the end we'll almost certainly get a new sku the XL or legendary edition as well.
Rhonda Rousey celebrity promotion will appeal to a sector of the market not traditional engaged with the franchise. Same goes for what ever future guest characters they announce.
WB have supported NRS financially more than ever before in this marketing cycle from the reveal/launch event onwards. How many games get that kind of support from their publisher!
It's also launching in a brilliant window. There's few third party games releasing and really only competing release wise is Days Gone. That been a Sony exclusive and little overlap in playerbase it won't canabalise sales much at all.
It's launching on more platforms than MKX (Switch) with better promised PC support.
From the positive reception from the media and the fc community everything seems to be going in MKs favour.
If it sells less than MKX... I'll main Rain