The original Blue Train electrification/route modernisation programme had an even higher frequency of up to 20tph, I believe, but it was a different network. As far as I'm aware it meant that the current Argyle Line services running through Partick went through Queen Street instead, and there was an additional branch (Bridgeton) to serve on the eastern side. The opening of the Argyle Line means that the same service frequency can be provided to the western side of the city as before with fewer train through Queen Street and to the east, while there's a smaller capacity relief heading westwards towards Bridgeton.
A big problem with the idea of filling up the tracks to Airdrie is that it would then slow down Airdrie-Bathgate services and make them less useful. There's more of an ability to speed up services in the west due to the Yoker loop and the turnback/branch provision at Milngavie and at Dalmuir. On the eastern side there's no (effective) dynamic loop, turnback or branch between Bellgrove and Airdrie. Branching off more services at Bellgrove is now more feasible than in the past as they can now go to Cumbernauld or beyond, rather than terminating short so close to the city centre that they're not competitive with buses or walking. Reopening the Bridgeton branch is now infeasible and not worthwhile. Before A-B opened some services terminated short and turned back in the Shettleston loops, but with A-B there aren't many trains that you can feasible turn back short here without disadvantaging somewhere else. The chord at Coatbridge Sunnyside could be used to turn back trains but it's probably too close to the turnback at Airdrie. It might work if trains went out of service at Easterhouse and then ran fast but the you're missing out on the demand from the Coatbridge area unless you build some expensive, complicated platform arrangements on the chord line. I think this idea would be much more desirable if you use it to send trains down to Motherwell or even onto the newly-electrified Shotts line, possibly with a call at a new station built near the Eurocentral estate. Even then it's unlikely to speed the timetable up by much at all.
Another idea could be to rebuild a lost chord at Sunnyside heading up towards Gartcosh. Then services from the A-B line could head up through Stepps, possibly justifying the construction of a direct Garngad chord so that they have a clear run back onto the North Clyde at Bellgrove. This chord would prove especially useful when a cross-city tunnel is built, as then A-B services could head through the tunnel and run south and/or west of the city.
Still though, it won't be easy to shoehorn faster regional services onto a line engineered as an S-Bahn. NR think that it would be worthwhile to build a third platform line at Bathgate to allow the extra 2 A-B services to be overtaken. The A-B stopping service (which will get even slower with demands for a station at Plains) could effectively be split at Bathgate, with the train waiting to be overtaken and then continuing on. With work a similar arrangement could be possible at Airdrie too but I don't see it being worthwhile in addition without other things being done elsewhere along the line.