If you ask me, the combos generally make sense. There are spells that you can use in between the combos when they come off cooldown. The whole point is to weave your CDs in between your combo spells, have one button to spend what 3 buttons usually do would make the most boring gameplay in my opinion. If you can consistently hit cooldowns and other things in between combos you can and will be a higher dps than someone who just clicks 1-2-3 then uses other spells.

Here is a prime example:

GNB standard single target rotation will consist of using Keen Edge, Brutal Shell and Solid Barrel. Brutal Shell gives health and shield and Solid Barrel gives you ammo for using other abilities. If you had already started this combo but you were full of ammo you can simply use Brutal Shield to mitigate damage, hold that combo in position 2, cast other spells which doesn't break the combo to spend an ammo round and then finish that combo to have another shell to use straight away. This is perfect for having the most amount of ammo rounds within your Damage up cooldown. It also allows you to meditate damage faster when you need it without wasting ammo.

RDM one isn't perfect but it's easy to fuck up. If you accidentally press something else you've just wasted white and black magic, adding insult to injury your big hitters are straight after the combo being done successfully. RDM without the combos would literally be the easiest class and would require no thinking at all. Not that it's difficult now but removing said combos would make it a really boring class.

I have always been an advocate that simply having better coding for their macro system would cut down keybindings and button bloat by a ridiculous amount. I want to be able to use modifiers in macros instead of having to literally have a separate action bar and keybinding for what should be able to be in 1 button using a macro and a modifier. They have one of the worst macro systems I have seen. Their mouse over system is delayed and completely lacking all control, their targeting systems aren't great with error codes and sound effects for errors, you cant use cast sequences and the biggest problem is, of course, you can't use modifiers.