They definitely could. I gave them each a bonus action and reaction instead. I'm trying to find ways to make interesting and engaging monsters without always having to resort to legendary actions. It is still a work in progress
Although, I have been thinking about a reduced legendary actions where there is only one or two actions. That might work well for higher CR demons and devils. Of the two, I think the Pit Fiend is a better fit as I see them as the more versatile fighter compared to the brutish balor. I will give the Pit Fiend another spin when I get a minute.