Ecumenical is supposed to be a flexible school. And It is. It has healing, anti-healing, reflect, poison/dot cure and several different damage spells. Its not supposed to really excel at anything.
Necromancy is a Damage school with a bunch of utility spells based on an offensive playstyle. Casting while moving(albeit slowly). Blinds, Selfheal through damage, relatively short range and they have area denial, be it from summoning or the darkness spell.
Elementalist is a Defensive focused school with loads of support spells for out of combat use. Changing temperature, weather, erecting obstacles but also some damage abilities too.
Animism, even though we've never had it released, we do know what spells they had planned and cross referencing them with UO you get a; short range buff+debuff school with some melee range healing thrown in and some nature based utility spells tailored towards animals. Heck, they get to control wind direction! which was going to be useful in MO1 since wind DID impact archers and projectile magic. Oh yeah, thats another of the strengths of ecumenical, its magic is not projectile.
Then there are all the other schools, like staff magic, dragon magic, mentalism and illusions. They cover many other facets of what you could want for a mage with different playstyles, mobile vs immobile. Schools like Elementalist are basically LESS mobile than ecumenical due to pre-casting being a ecumenical thing, while necromancy is semi-mobile with cast while moving with direct cast upon completion. Staff magic would technically be the most mobile school I think.
No, you all can't have everything OP shoved into one school, if you want that, try modding skyrim.