Let's work through this logically.
Current operation
They currently run 5 return trips a day.
The trains have on average, an hour layover in Waverley, in the east end bay platforms. The actual duration ranges from 50 minutes (19:11 until 19:58) to 75 minutes (10:10 until 11:25)
None of these layovers are long enough to do a return trip to Glasgow, so that means either more trains, or rewriting the timetable. Either way, nothing in the timetable gives us any evidence assuming how many trains might make it to Glasgow, or which station, or at which times. So we work from scratch, and assume a new timetable, or more trains, or both.
Potential demand
There are currently direct drains between York, Newcastle, Edinburgh, Motherwell and Glasgow Central, albeit once per day in each direction through each of XC and LNER. The latter is going in the December timetable change, and the former is supposed to be running broadly two-hourly, however I suspect we've seen the last of that.
There obviously is a market for direct services between the ECML and Glasgow, simply given the size of the cities involved. That means there probably would be at least some passengers doing those journeys on any direct services that ran. The difficulty here is that Edinburgh is a much, much bigger traffic draw than Glasgow, at certain times of year up to a factor of ten.
There are 3tph at least between Edinburgh, Newcastle and York, and at least 2tph all the way to London. These services connect comfortably onto regular ScotRail services across the Central Belt. There is also a direct service up the WCML, from Glasgow to London, again connecting onto regular services across the Central Belt.
Commercial viability
This gives us a potential market share for Lumo to capture, which basically amounts to cross-Edinburgh passengers. Given that Lumo is severely limited in where it is allowed to stop, this new market share is those passengers from Newcastle/Morpeth to Glasgow, or vice versa. I doubt in the strongest possible terms, that this is enough for a purely commercial enterprise to make a profit.
There will be some people who would pick Lumo all the way to London, because it's cheaper. There will also be the occasional person who jumps on the Lumo between Edinburgh and Glasgow, and the ORCATS raid that comes with this. These two groups will be where the profit is found. For either of these, heading to Central vs Queen Street doesn't really make a difference, as one is faster but the other is less well served.
I suspect, if this does go ahead, we will see one or two of the five daily services extended to Central, as pathing will be easier, and that they will be full with cheap advances from Glasgow to London. But that's a three hour round trip, and that means at least one extra unit. Lumo has already proved that it's profitable on a very lucrative axis, and the logical move would be to increase capacity on the services it already runs, rather than push for an extension with a dubious at best, and doubtful at worst, business case.
The ideological factor
Someone pointed out earlier, and I apologise that I can't find the original message to link, that Lumo is an experiment in commercial competition on an already profitable axis. Its the first open access operation that hasn't been held to the Primarily Abstractive test, and I expect that this is why the Glasgow extension has been mooted.
If Lumo has decided that the current government is ideologically opposed to public monopolies, they may have also worked out that by extending to Glasgow, they can raid all those nice, juicy advances. Suddenly, we have a business case. If it happens, which I actually expect isn't beyond the realms of possibility, this will be why.