Most bus routes in London, or other cities with high bus patronage, have headways between 8-12 minutes.
When the wait time excess that, the attractiveness of bus service decreases a lot. While an hourly service is acceptable in rural areas, very few, if none at all, will use such kind of service for urban travel.
A single decker being full on a route capable for double decker operation is a clear example of wrong allocation, no matter how frequent the route is.
8-12 minutes is a very good frequency, compared to hourly, or two hourly services in more rural parts of Dorset. There are even worse frequency levels as a lot of the shopper services were stopped around 5 years ago.
I don't agree that a single decker being full is an incorrect allocation. As previously mentioned Yellows operate a number of c40 seater vehicles in fleet as does Morebus. With the standing capacity on offer, they can move 70 people at once. They're cheaper to purchase, and cheaper to operate.
One problem I had when working at Yellows was the tiny number of 29 seat Enviro 200s/Solos we had in fleet (around 10), These would be put onto double decker workings and would struggle to deal with the passenger loadings, whereas a Dart SLF would be able to move 60 pax in the peak hours coming through Winton, so you'd get away with those as a substitution. After a few months of 5 Solos sat in the yard gathering leaves someone decided to sell the vehicles, as we had no use for small buses from a scheduling perspective. They'd been bought in to run Dorchester Town Services as part of a stillborn project in 2011.
Yellows had a long history of double deck operation, but the Transdev/RATP management disliked such vehicles and preferred instead to purchase larger single decks which could stand 25-30pax and seat 40. That said, the 1b/1c and 5a/b were booked double deck lines - so around 40 of the 120 peak vehicle requirement needed to be deckers. So post 2005 the annual investment in double decks more or less ceased and attention paid to single decks, which make up the bulk of the bus fleet today.
I'd say from experience of the network and planning it that the fleet composition is the right one. Besides from reducing frequencies to accommodate more double deck workings I can't see why they'd want to alter that fleet make up - the one corridor Morebus never challenged on was the 5a/b to Kinson through Winton. Problem with cutting frequencies in a competitive environment such as Bournemouth and Poole is it gives ground to the other competitor, so you'd not want to do that unless forced to.
Most of the vehicles used on the 5's were the Y/02 reg batch of East Lancs Vykings, allegedly mechanically superior to a Trident, but being worked 15 hours a day for 12/15 years took it's toll and they frequently failed in service. Repeatedly. Only late into RATP's ownership did new double decks arrive, which didn't resolve this rump of 12 to 15 year old double deckers which needed replacing. That took place late in 2017/early 2018 just before the sale to the management team and only then did the surplus vehicles in the fleet (around 25 over the peak vehicle requirement) get disposed of.