Five souls is more limiting than it sounds considering that some souls are more "specialized" within a given archtype. It will make people feel pressured to make damn sure they have those. AKA many mages will feel strong pressure to pick up chloro, dominator, and archon so that they can fill those specialty roles if they find they need to, this will only give them two souls to pick from.
That and by limiting souls people will be more keen on picking the "right" souls and having cookie cutter builds because they will worry about eternally shafting themselves on the "wrong" soul that ends up being gimped at some aspect of the game. This will directly discourage experimentation and exploration of the various souls available and hurt diversity, not help it. You will get everyone loading the exact same souls so that they are "safe".
And then of course if souls get rebalanced nerfed/buffed someone might find their character's playstyle broken with no recourse because they've already picked their souls etc. Now they must re-roll an alt, not because they wanted to but because they were compelled to in order to feel viable.
An open system encourages experimentation and diversity and allows people to try to find a mix that works right for themselves without fear that they will screw themselves over if their typical build just doesn't work in particular environments.
Will there be "standard" min/max builds that many people will copy? Sure...but limiting souls will only ensure that nearly everyone will use those builds 24/7 because their soul choices are locked vs. branching out much of the time.
There is absolutly nothing stopping someone from making a ton of alts if they really want to level up another character using differnt builds, so that argument is utterly pointless. People who want to roll a ton of alts can do so, people who don't feel like it won't have to as much as they would in most other games. That's a win win situation, period.
Personally the idea of a game that allows me to focus more on a few characters sounds fantastic.
The argument of having access to more diversity somehow mystically making the game easier is just silly. The difficulty of content is what it is. Letting people play a game in a manner that is the most fun for them has jack-all to do with it. There are a couple of threads now with people saying "you just want to be able to do everything!"
This is silly, in every MMO, everyone already can do everything (skill of themselves and/or their friends limiting of course). Its a matter of how much time they want to put into it in most cases. Everyone already has access to all end game content if they choose to pursue it. Everyone already can play any character they want. Trion is simply trying to eliminate tedium and situations that cause people to feel compelled to grind over and over. Grinding for most people isn't fun, sure we did it back in the day when there wasn't anything else out there, but even then pretty much everyone throught that the time spent to camp the ancient cyclops was rediculous.
Most people play games to have fun. Having fun doesn't mean someone isn't dedicated, it doesn't make them a less skilled player, and it certainly doesn't mean they don't love and thrive on challenge. What it means is most people recognize that grinding is boring and will avoid it if they can because being bored defeats the whole damn purpose of a given hobby.