Good day. This might be a stupid question, I'm currently have more lamps than the ballasts, and the ballast gets hot which is what ultimately reduce its lifespan I believe, or running an old near-eol lamps which draws more power than the ballast output resulting in the ballast working harder that it should, so it gets even hotter. So my question is does adding heatsinks on top of the ballast and even going as far as adding a fan on top of the heatsink fins help extends the life of the ballast? Some leak ballasts are way too precious nowadays.

Is it going to be useful or it is totally useless that I can forget about it as it doesn't do anything?

If you would want to pursue it with a plan then i would, but the effectivity may be varied. Most ballasts are designed to work while hot, so an occasional run should be fine. However some are designed for passive airflow on all sides for cooling, and if you restrict the airflow then life would go down. The least you could do is put it on a stand with space underneath. The main trouble would be capacitors as they are a great deal more heat sensitive. GE, westinghouse, and a few others all had to rework their 700/1000w mercury streetlights with fins on the top to put the capacitors in and cool them. Nowadays cap tech is better, but i would make sure its got a few centimeters between anything that gets hot and also have good airflow for it. I know other members have built fans for cooling ballast, if you want to do that then by all means go ahead, will only take a little bit of time an is an extra layer of longevity.

When the temperature is within the design limits, that means the ballast should be able to just reach its rated life. Which is not that long - usually 30k hours or so (~ 10 years of service). So virtually any older, used, ballast is past its formally rated life.
So in order to operate that vintage equipment, you should handle it in a way it will last way longer than was the official rating, that means mainly operating it below its maximum rated temperature.
The "each 10degC means lifetime divided by 2" rule is the most dominant factor here and the fact most ballasts are operated 20..30degC below the max temperature rating is the only reason why they are still working after 30 or 40 years of service.

So adding the heatsink does make sense, same as adding other means of cooling. The used ballasts may have just few percent of their rated life left, but keeping the temperature low means those few percent may still cover quite a few extra years of service.

The ballast will usually be marked with the temperature above ambient above which it should not rise. This is usually quite generous, maybe 60 or 70 degrees.

It can possibly help with cooling to add a heatsink, some higher wattage ballasts come with them already. A fan can help as well, we made up a fan/heatsink arrangement from the cooling fan of an old computer, see here.

I used to have lights with magnetic ballasts for houseplants in my bedroom...
The ballasts always got too hot to touch, that's just the nature of a 2xF40 ballast!
Also in the summer, the room (especially up near the ceiling) would get well above 90F basically every day.
I had fans to blow air across the top of the fixtures to keep them a bit cooler (try to prevent overheating), just because it was so warm in the room..