The microtransactions are optional.
No, really, actually optional.
You can buy a shining new weapon for Real Money (via "Platinum"), or you can do missions and save up the credits to buy the blueprints and then get the resources to build it yourself. It'll have the exact same stats, it'll just take longer.
Quite honestly, I have my handful of weapons and warframes that I play with, and I have no ambitions to unlock eeeeeverything, so what compelled me to buy Platinum at all was the fact that I had played a hundred hours for free and wanted to support the studio/programmers.
Oh, and i wanted a nice bobblehead of my favorite warframe on the dash of my Liset.