The bus actually went exactly where the local community council wanted it to go and they worked with the local operators/SPT to design an appropriate route and timetable. However, like most similar scenarios, people simply don’t use the bus as much as they initially claim they will.
I have no doubt the community felt like Braehead was the place underserved given train links but there is paying £3 plus on the bus instead of driving to thousands of free parking spaces so fully agree on the second part!
I think the council papers suggested that a link to Erskine was an option and I think additional subsidy to change the 521 might have been a better use - it would have gone further and meets demand for High School for school pupils, local leisure centre and local shopping centre - although I still don't think it would ever work commercially.
I guess in any future franchised system you could extend the 23 bus to suit, and this would also meet demand on the Glasgow-Govan-Braehead-Renfrew corridor.
I disagree regarding desired destinations - the 514 provides a public transport link to Bishopton town centre and Braehead and to Paisley, Glasgow and Inverclyde through rail connections at Bishopton station. These are likely the most popular destinations. It's just that buses are generally an inferior good - demand falls as income rises, and with house prices starting around 250k in Dargavel, the pool of potential users is very minimal.
On the other hand, usage at Bishopton railway station has now surpassed pre-Covid figures, increasing at a far faster rate than other stations on the Inverclyde line. It's likely a lot of people in Dargavel have simply chosen the train over the bus, hence the end of the 514. To be honest, you're probably right that it was a waste of developer funding. At least they gave it a try. I wouldn't be surprised if the M7 went the same way - properties seem to be even more expensive in Maidenhill than in Dargavel.
I don't think I've ever considered Bishopton to have a 'town centre'. It isn't that big - most of the houses in Dargavel are still within 10-15 minutes walk of the station and you can probably walk end to end through Bishopton in 20 minutes. Can't see enough people making a bus journey within Bishopton to add up commercially especially one that involves an infrequent bus on a non clockface timetable and paying a fare for the privilege when you can walk it - or drive to the small retail parades.
Like you say the train takes most of the key destinations where people want to go. This limits bus need and makes it hard to make anything commercial stack up. Bishopton has never had that much of a reliable commercial bus service going back years and it's for a reason.
I get the 'inferior good' and hard to serve new housing on edges of towns for a variety of reasons, but I'm more positive about bus use but it needs to be the right service for the area and not just trying to ensure it's just covered by a bus.