The routes are 2+ hours long, but I suspect most actual journeys on them are under an hour.
Apparently, the average journey time on the Thameslink routes is well under the hour, - more like 35 minutes. Apart from the core stations and the key outer stations having high footfalls, look at the figures for the metro stops, the routes have a consistently high usage.
The interiors of the 700s are awful full stop, the lack of WiFi on the 8 car trains another example of their poor spec when compared to the refurbished trains elsewhere on the network and indeed a lot of recent buses
That's your opinion of their interior. They have now been in service for over 4 years and are generally pretty reliable now the initial bugs were cleared. The fact is that regular passengers know they can turn up at the station and board any train* irrespective of the time of day, they are fast, relatively smooth even over rough track and rarely taken out of service.
Comparisons with trains provided for services elsewhere in the UK is pointless, the 700s are designed to move the maximum number of passengers over a highly tuned but absolutely limited infrastructure. Furthermore, that demand is not a simple gathering passengers on the way into the city and delivering them at a terminus, (then the reverse in the other peak), it is a distribution service from about 20 miles outside central London, with frequent major interchange stops, e.g. West Hampstead, East Croydon, Wimbledon etc..
* Not counting the current dysfunctional situation during the pandemic. Up to March last year, they were coping well with the ever increasing demand on Thameslink routes.