Speculating on X’s gross is completely futile right now. Way too many unknown factors. For starters, we don’t know how long the gap will be, which will have a huge effect on how well it does. It’s not going to be massive like TPM and TFA is the gap is only a few years and there’s not time for people to really miss SW.
And more importantly, IX has the uphill battle of appeasing the bulk of the fanbase, or else it will be the first trilogy to end on a divisive note, and that won’t be good for X.
Finally, one factor that does seem pretty clear that will be a hindrance to X for sure is that we will officially be done with the OT characters, so there goes a very important part of all three trilogy’s appeal.