Low passenger numbers mean the reason for electrifying Maryhill is operational convenience more than anything else. That's not to say it's an irrelevance though. When ScotRail will be trying to pour a gallon into a pint pot with the future Queen Street HL timetable, any 'quick win' which unlocks other improvements is going to be looked upon favourably. With view of the low passenger numbers I very much expect the work to be done via a major blockade, like much of the Paisley Canal wiring. It would be much cheaper just to bus passengers for a few weeks and have all hands on deck rebuilding bridges and lowering track than it would be to have a protracted set of works over weekends and bank holidays. Actually, a thought occurs that the cheapest time to electrify it might be at the same time as work on East Kilbride/Barrhead. These lines aren't going to be so easy to close, so there will be lots of resources needing paid for that will only be usable for a small proportion of the time. If the kit can make its way back and forth between the two sites, you could have it work on the Maryhill line during normal times and then on the EK line whenever a closure can be arranged. If there are any remaining gaps maybe they could just finish the job and wire up the City Union while they're at it?
I don't imagine they would electrify EK without doing Barrhead in the same job. With only a few kilometres more wiring you can convert all of the stopping trains on the line to electric, sharing stock and simplifying timetabling. Barrhead is as far as you can go without having to think about the much bigger project of redoubling and/or wiring to Kilmarnock.
How about re-routing Maryhill services to Milngavie rather than Anniesland? The 3-car EMUs wouldn’t be carrying so much fresh air and Milngavie would get 6tph without requiring any additional capacity at Queen Street.
Kelvindale would have to be closed but you could build a replacement on the WHL to the north of the existing station.
What you're proposing is the reversal of what happened when the Anniesland extension opened. Before that, the DMUs went onto the start of the Milngavie branch to turn back. When Larkhall reopened they added another 2tph through Central low level and Milngavie was the best place to terminate the trains. The new turnback siding west of Maryhill just so happened to be long enough to stretch all the way to Anniesland and have a new station on it at the same time.
Moving forward I really don't see them reverting this decision, as the factors in play back in 2005 are even more pressing today. You would have to up the Milngavie branch to 6tph, requiring full redoubling back to Westerton, and then you would have your Maryhill trains cross both running lines on the flat. It would mean that problems on the North Clyde/Argyle lines would have an impact on the high level trains, which can't be good for reliability.