26,000 miles over 3 years is about 8,000 miles per year which is the UK average.
You can drive once per week and do that mileage, or drive every day. If you drive every day, it is still 20 miles per day, which even in stop start traffic is enough to warm the car. The stop start system does not operate when the engine is cold.
The problem with the carbon build up is more of a design feature of these engines. For sure they were not designed to be driven in stop start conditions but this is how the roads are nowadays.
The expression driving on the motorway is a bit misleading nowadays. The motorways have the highest speed limit in the UK but try commuting to work and you will find out that you spend a lot of the time parked with the handbreak on.
Then you try again on the weekends and it is marginally better. Most people use the cars for commuting to work and driving away on the holidays. On both scenarios they use the motorways only to be parked idling to keep them warm or cool.
Motorway driving used to mean higher speeds which required higher revolutions. This had the potential to take away the accummulated dirt. But as nowadays the motorway traffic is slow moving, the expression should be replaced with something like drive the car long enough without the need to stop.
The speed and the revolutions do not matter as long as you can keep the car moving at the highest gear. Then you have a chance to burn out the accummulated dirt.
But the problem here is that the natural burning cycle of these engine result in excessive carbon. If you only use the car for commuting, shopping and holidays, then you cannot avoid this dirt accummulation.
Luckily, most drivers do not have this problem because they buy the car in finance and swap it with another every couple of years. For the unlucky drivers that they do not do that, the decoke is a good option to cover for the bad design.
The other option is to drive away on the weekends and get some fresh air but most people do not do that. If people were giving a chance to the car to drive some considerate time without stopping, then it could clean up a bit but the problem would return with the standing traffic reality.
When the current generation of E Class was introduced, the reviews mentioned that the engine sits at 1,100 rpm at 70mph thanks to its 9 speed gearbox. So in the traditional thinking that driving the car on the motorway will clean up the engine, this will not happen because of the low revolutions but because of the time the engine will keep running non stop.
Mazda never got it right with the diesel engines but there is no way they will admit that.