With a bit of hypermilling, I've acheived 857KM on my 2.0D Saloon 2002 with 15%+ tank remaining that when refilled and calculated came out to be 21-22 kmpl. I get at least 14 in city which includes idling on A/C and exploiting torque band of 2c most of the time, all the power present between 1500-3500 rpm.
And about maintenance, it has a bit larger oil sump than petrol but buying oil in bulk proves cheaper in longer run and delo gold can be used everywhere, generators, suzukis, almost all pre 2005 cars no need for 'xpensive synthetic or low viscosity motor oils.
Diesel labs can do all kinds of engine related work but they cost more so people opt to get the mechanical work done somewhere else and only tune the engine from labs.
token tax for more than 5 year old vehicles is significantly less.
parts are expensive due to durability, you can't overhaul modern petrol engine's properly in Pakistan while 2c runs at least 2.5lac on genuine ring pistons with just oil filter change and 3.5 lac+ if one maintains it properly.