This is the tension between rules and rulings though. When a spell has a single defined feature and parameters, it acts as a single tool. The problem comes quickly that unless every problem is solved with the few tools you have on hand, people will naturally seek more and more tools. If healing hp costs an action and must be a touch distance, naturally someone will want weaker healing at range using a bonus action. Now there are two healing spells to track. Want to heal more than one person? We move up to four. Etc.
But what is the solution? A single healing spell that does all that stuff depending on caster parameters? You've just made casting more complex for player and DM? Does all that stuff at once? OP. Only one effect exists? People will reinvent them anyway. Simple=limited only works for onboarding, but people quickly seek more complex options even if they get overwhelmed by them.
I agree in principal, but have a different take in practice.
I don't read this poll as "I want a caster a new player can create at 20th level and it's simple". I read this as "I was a caster the stays simple to play for a new player starting at 1st." The difference there is that once some foundational mechanics have been absorbed, you can add additional mechanics as long as you keep them simple as well.
For example, at 1st level a caster just needs to worry about spell slots. At 3rd level, they suddenly need to worry about different scaled spell slots, 1st vs 2nd, This is additional complexity, but it's a minor extension to what they have been practicing for two levels so shouldn't push up the complexity to running, because at 1st level they were a new player with some small X of mechanics to understand, but then context now is that they are no longer a new player, but a player with experience of playing two levels of the class, so giving them some small X of additional mechanics to understand is still keeping it simple.
So in the case like your example, where we introduce a Mass Cure Wounds a number of levels later, that can be a reasonable outgrowth.
So simple is both in onboarding, and in ongoing, with the ongoing under the context that the player has completed their "apprenticeship" of onboarding. If people want more, there are always the non-simple caster classes.