Souls are ABSOLUTELY the big limiting currency at high levels of play. I feel that pinch myself every day!
There are about 8,000 “easy” souls to get from quests & challenges. That’s enough to get new players a few level 15 cards, which is the threshold at which they can be competitive in PvP. That allows them to start earning glory at a reasonable rate.
Earning souls beyond that is much slower - several weeks to level up a team of 4 new cards. (This is slightly mitigated these days by the hero buff, since many teams are now 3 troops & hero).
At very high levels of play, arena runs are probably the fastest way to earn souls - Celestial armor will get you 1,000 souls for winning 8 fights. Farming challenges with Valkyrie, at a difficulty slightly above normal, can earn you 100 souls a fight - which is comparable.
But the intent is that most players never even get close to “max level” for all their troops. Games like GoW are at their best when the scarcity of resources is nicely balanced to the rate at which new content is introduced, to give players interesting allocation problems with unique and satisfying solutions.