Unlimited unit selection just isnt necessary, you rarely have 20+ units of 1 type unless your massing flying machines or archers. Increasing unit selection to 20 would be more than sufficient. The limit to unit selection was not because of technical limitations, earlier games had unlimited unit selection but Blizz chose to make WC3 with limited selection to encourage player to get smaller armies. Of course, it should be an option to allow unlimited selection for custom games.
A better solution to this debate would be to allow players the option to pre-set their control groups per unit type, so you can just press 1 for melee 2 for ranged 3 for casters etc… I posted about it several times before
https://us.forums.blizzard.com/en/warcraft3/t/pre-set-control-groups-in-option