Don't get hung-up on the number of seats. MPV stands for Multi-Purpose-Vehicle.
What that means is that manufacturers have placed function above form to squeeze as much interior space as is possible from a car's floorpan. The simplest way to do this, is to make it higher, and more regular-shaped than the base car it is developed from.
Vauxhall Meriva vs Corsa, Vauxhall Zafira vs Astra, Citroen Picasso vs Xsara, Citroen C4 Picasso vs C4, VW Touran vs Golf, Ford Fusion vs Fiesta, Ford Focus C-Max vs Focus, Ford S-Max & Galaxy vs Mondeo, Fiat Multipla vs Bravo, Honda FRV vs Civic, etc, etc.
Many of these don't have 7 seats. But they are all higher than the cars they were developed from, because by adding height, not only do you increase the load area (giving it the Multi-Purpose aspect), you can get the seats closer together because there is more headroom - this is why a Zafira can fit 2 extra passengers into an Astra's floorpan (for example).
MPV is not a derogatory term, so don't be offended by it.
P.S. Driving dynamics do suffer, as not only are aerodynamics a casualty by adding height (the frontal area is increased), so top speed & economy diminish, but by raising the centre of gravity, handling/roadholding also takes a hit (not many MPVs setting sub-nine minute laps of the Nurburgring, for example).
MPVs were developed (with the exception of Chrysler & Renault) as plush minibuses - based on vans (such as your Serena which came from the Nissan Vanette), however, as customer expectations grew, manufacturers realised that something a bit better was required - hence the switch to car-based products.
Van-based MPVs still exist; Fiat Doblo, Citroen & Peugeot offer one, and so does Renault with the Kangoo - all worthy vehicles, but priced at a level one would expect given their origins.