BS.
No it's not.
As I said, it's who knows how to use them best, and by that I do not mean the effects, as you say, they are fixed for each spell. I mean by which spell to use when. Someone with vast amounts of spells might get carried away and cast spells that make them tired very quickly meaning the other person, with just a dozen or so spells, could take advantage of the situation.
The one who has the most spells may well have a huge advantage but
only if they know how/when to use them properly. Someone with a vast quantity of spells who does not know how to use them best will probably find having so many options a hindrance rather than a help.
I think the problem here isn't just that someone who comes along later will have fewer spells because it's a younger character. The problem is that the people here right now have the chance to research lots of basic spells, spells that are simple and obvious, but that haven't been laid out by the GMs from the start. They get to make them up, and then their characters know them.
The spells being done now, before the game starts properly are, as far as I'm aware, going to be kept in the game but the characters will not necessarily keep knowledge of each one once we go active.
When people come along later, they will not be able to research these basic spells, because once one person has created Fireball, no one else can ever learn a Fireball spell except from them! (or, obviously, in a chain leading back to them)
As has been stated elsewhere, if/when Councils are up and running, if they take similar form to the past the keep their own grimoires which are made available to all members of the Council (possibly split at different levels). So a new player, if they joined a Council, would have most of the basic spells made available to them through that.
This isn't a game about people spending most of their time away from other SpellCasters, there should be lots of interactions and opportunities for trade/teaching/learning.
If there do seem to be problems once we're underway, we can look at addressing things, but the basic spells should be relatively easy for people to get hold of once we've been going a while.
