as explained above, your character is a kid in your ship. the warframes are tools that each have four abilities. the tenno (kid) also has focus system. you can max them all out and own them all and use them interchangeably at will.
the difference here, is that the abilities are unique but can still be owned by owning the warframes. here, the skills are generic and should be available to everyone (are available, as of the current concept, with limits).
also, you can as stated above, max out all the focus skill trees and use them when needed as needed.
the equivalent in boundless would be to allow players to max out all skill trees, but only have select ones active at a time, i.e. active skill sets. so, youd swap your skill set to explorer builder warrior to go out and settle new areas or farming resources, then swap back to crafter machinist engineer or w/e when at home working on tools and or your home.
if it were like this, i would not be losing interest in an otherwise fun and interesting and beautiful looking game.
and while i appreciate that some games do some things better than others, this is 2017 and imo certain games deserve to set new standards. limiting your players in the way that you can do this but if you do you cant do that! seems old fashioned and backwards.
let us do it all, as good as we can, as much as can be, if we are willing to invest the time and effort into it. and by time i dont mean crafting/wait time.
also, just to add, i never said six classes. there are five focus trees, skill trees, used by the operator. iirc there is something like almost fifty warframes (some of them are 'prime variants' that have the same skills but are different account rank xp and have a minor buff here or there), about 35 of which are unique.
not to mention three types of pet, kubrow (dog) with five breeds, kavat (cat) with two breeds, and charger (infested abomination) 1 breed.
and you also have i think its eight or nine base companion drones.. djinn diriga carrier wyrm helios dethcube shade taxon, three of which have prime variants (again, just slightly better stats here or there and separate mastery)
every warframe can use any weapon, and sentinels (companion drones) can change weapons too, each coming with their own but able to use the others (with the exception of helios's weapon only usable by helios but he can use any other weapon instead)
the freedom to customize is so amazing in warframe and they arent limiting you based on your choices