Why 2 4 6 8 10 rather than have the cost and 1 2 3 4 5?
To be clear. A level 1 Psion has 2 points. A level 20 Psion has 21 points.
A slot 1 spell like
Magic Missile costs 1 point.
A slot 9 spell like
Wish costs 9 points.
And of course, the highest slot possible equals (level+1)/2. A level 10 Psion can only cast spells of slots upto 5 points like Telekinesis, namely = (10+1)/2 round down − so
Wish remains uncastable until the highest tier.
The big problem with power point systems is how spamtastic they get, especially in combat.
Keeping the spell point pool small means, most of the time, one can only cast two highest level spells before needing to rest. At higher levels, one tends to reserve spell points in case of soon-to-come combat encounters before a rest becomes possible.
When advancing to higher class levels, to spam lower level spells is a reasonable choice but to cast a couple of higher level spells is also a reasonable choice.
The small short-rest point pool is self-balancing.
There are normally optimal spells that you just cast again and again. Eldritch Blast is fine - but limited resource versions make the game less interesting because those are your shining moments and shouldn't be all the same.
That narrowness is because of the spells themselves. If the 5e spells themselves are imbalanced, so that in the same slot one spell is strictly better than other spells, it isnt the players fault for choosing the better spell. The problem is the 5e design.
The 5e spells are in need of a drastic overhaul, to calibrate all of the spells so that every spell is roughly comparable in value and usefulness to the other spells in the same spell slot.
Hopefully, by the time the Psion becomes a class, all of the spells in 5e will balance better with each other.
And short rests don't really work in 5e.
Short rests work well with spell points.