Considerable code and hardware compatibility. I have replaced a 16F with 18F due to last minute change in specs, needing more horsepower. The 18F452 and 16F877A are pin compatible.
I will bet you that a modern 89C51Rx2 is more "compatible" to a 1990 '51 than a PIC18 is "compatible" to a PIC12. Pin compatible they are as well.

Tools and chips by the same company. Limited third party chaos
as far as I know the "Keil" of the PIC is HiTech, hardly "the same company"

Scalability, really 6 pins to 80 pins from one company
Not much different, the '51 comes in 8 (89LPC091) to 100 (8051f120) pins. If you insist on "one company" (who cares, what is important is "one architecture") SILabs offer 10 to 100 pins.

In India, even though a small market, top class sample and sales support.
Good for you. Since you compare, have you tried Atmel, Philips, Silabs, Goal, Infineon, TI, Intel, ST Micro, Analog Devices, Cypres, Dallas, Winbond and the rest of the '51 manufactureres. I see the span of manufacturers as one of the largest advantages of the '51.
What are you going to do if Microchip change distributor in India to a (insert derogative here)

Erik