See people pick Thermite because his utility is too good + claymore and good gun. But you can do without him because of Hibana and there are operators with similar/better guns, so it's not like old days when he was absolutely must. However Ash's utility is far less useful as any walls worth breaching are usually reinforced and her launcher is better used to get rid of shields, barb wires and the occasional kill from blowing up the floor underneath someone.
Unlike Thermite however, people don't pick Ash for her utility but because of her tiny hitboxes and 3 speed mixed with her good fun. Often it almost feels like cheating going up against ash due to how enormously hard it is to hit her and how good her gun is at killing. I don't get this feeling when going up against Thermite.
Though the one thing I'd like to be looked into is Thatcher's emp, there's basically no point putting mute charge or batteries in most matches anymore because Thatcher can take them all out in one go from the outside without even 1% risk. That bit at the end right there is why I want it to be looked into, it takes no skill and zero risk to get rid of all these devices as you can emp everything from the outside. Compare this to the other operator that's capable of disabling electronics i.e. Twitch who actually has to risk her drones and manually get rid of each devices one by one from the inside.
I guess you could say that one of the reasons Thermite is so prevalent and powerful in terms of utility is because of how easy, safe and quick it is to disable everything using Thatcher.