As a local (to Watford) I would always lay a large chunk of the blame for the terrible bus services (both network and operationally) at the door of Arriva . . . but that's not the only reason.
The local council structure is that Watford Borough is almost totally surrounded by Three Rivers District, and on the remaining side by Hertsmere Borough . . . sometimes only two miles from the centre of Watford. All three councils have different attitudes to bus transport, and Herts CC haven't been able to broker any sort of agreement between them (not that they've tried especially hard).
Additionally, the London Buses influence shouldn't be under-estimated. Although only one corridor with two routes, the cheap fares (and free for kids) since 2005-ish have influenced local attitudes ("if Bushey can have it, then why can't we"), such that locals won't even contemplate using the bus, even if it is actually convenient!
The fiasco that was Arriva Click, where Watford Borough spaffed something between £2.5m and £4m (depending which Watford Observer article is consulted) on a DRT operation that was restricted to Watford residents only, so residents of Croxley, South Oxhey and North Bushey were denied usage . . . and starting it in July 2020, when social distancing was still all the rage . . . bound to fail. I've calculated that another 10-12 PVR could have been funded on regular bus routes for two years with that money . . . and that would've done wonders for frequencies. Running a bus every 10 minutes between Holywell and the Junction? Bound to succeed!!
As far as competition is concerned . . . the last time was in 2007-2009, when Metroline had a go from a local base hard by Junction 6. Two routes were operated at 2 BPH, with a third ready to go, but regrettably Metroline took fright at the escalating losses and pulled the plug at short notice. Since then, only Mullany's Coaches has had a try (ut only in a limited way) . . . although their (Irish) parent company, again, couldn't see that slow progress was being made, and stepped back. There is only Uno left locally, and they only have a minority presence in the area.
What is probably needed is a minibus revolution in the town . . . coupled with a huge escalation in parking charges (classic stick and carrot). Is there anyone brave enough? No . . . not nearly . . . so the deterioration will continue. I honestly can't see Arriva remaining much longer . . . perhaps the answer is actually for them to withdraw, which will force Herts CC to do something substantial.
As far as Route 724 is concerned . . . with the revision to Route 725, I quite like the idea of running direct between Hertford and Hatfield, although 724 does perform a local function on the east side of WGC (I'm sure that 323/324 can be tweaked to cover). It would lose the link between Harlow and WGC of course . . . but whether that's important enough to worry about isn't known. Maple Cross? There is a contracted R1/R2 that might be tweaked in the longer term.
All in all . . . please don't forget that Arriva Kent Thameside are still the commercial operator in the area, and until (if-when) they leave, Herts CC are somewhat restricted . . . they cannot "force" Arriva KT to make changes, only suggest and propose. If only Arriva Midlands were in charge . . . !!