Multiple lines aren’t a problem for me. The really early stuff I had in the early nineties is long gone, but I still have a couple of 9.6 v Craftsman drills from around 1998. They function, but I am down to one battery.
Have one Dewalt 18v that I occasionally use, albeit with a current 20 v battery and an adapter.
Switched to 10/12 v Bosch and liked it, so I got some Bosch18 v and some more of the 10/12 v tools, all still in regular use.
Got a Skil 18v drill as a gift. It’s probablnearing ten years old but still works, so it’s still viable.
Pretty much standardized on Dewalt 20 v and Flexvolt around four years ago. Have around ten of those tools now.
Have some non fuel 12 v Milwaukee....meh.
Read too many posts here so I bought Milwaukee Fuel 1/2” high torque and 3/8” compact 18 volt impact wrenches, and a few batteries. Not impressed, so I just bought another Dewalt, an Atomic 18v 921 compact impact wrench. So far, it’s better than the Milwaukee.
That puts me at somewhere around two Dewalt, two Milwaukee, two Bosch, and a Skill platform in more or less general, regular use. I have tools in four locations (buildings), in three properties, in two states.
Lack of commonality isn’t an issue, but I will probably limit future acquisitions to Dewalt 20v and Flexvolt, Bosch 18 and 12 v, and Milwaukee 18 v.
I am staying away from HTP Metabo, Menards Masterforce, Flex, Kobalt, Ridgid, and Ryobi, all for no specific reason.