I can't speak for their motivation, but the what the Evening Standard (and Metro et al) printed was not related to the origins of the bus. They claimed they were dangerous to other road users. TfL disputes this.
Solving the overcrowding issue is more complicated than building higher capacity buses - you have to think about the speed of boarding, route layout, speed of navigating London's congested streets, fare evasion and more. In some places, the bus stops allow for two bendy buses or 3-4 regular buses to board at once, which means faster boarding on regular buses during peak times.
The new Routemasters, with 3 doors and 2 staircases, might prove to reduce overcrowding despite their lower capacity.
There was plenty of research behind introducing bendy buses. The main argument being that they transit the route faster overall because they have a much shorter dwell time at stops. The introduction of bendy buses was a rational evidence based decision, the campaign against them was mostly emotional hubris, which resulted in them being replaced early at vast expense to the taxpayer. The main reasonable complaint against them was that you were more likely to have to stand up, but you often couldn't even get on the bus (e.g's routes 73,38) before they were introduced so I don't know what's worse.
Solving the overcrowding issue is more complicated than building higher capacity buses - you have to think about the speed of boarding, route layout, speed of navigating London's congested streets, fare evasion and more. In some places, the bus stops allow for two bendy buses or 3-4 regular buses to board at once, which means faster boarding on regular buses during peak times.
The new Routemasters, with 3 doors and 2 staircases, might prove to reduce overcrowding despite their lower capacity.