They said there would be a constant stream, not rare. There were some issues with Exos a couple of times so we went a month or two without any. Statistically speaking, the more that spawn, the higher the chances are that they are a T7 with lucents or a new color. Waiting ? months for one T4 Exo to spawn with nothing new, is boring. 
They are also an integral part of the gameplay now because it’s the only way people can get certain supplies: resin, goo kernels, lucents (for farming, certain gear, concrete…)
The experimental Exos are what I’d call rare & 
special. Like the color-changing ones. [Chaltiin] --[T4 - Inhospitable Lush Color-Cycling Exoworld]-- [Inactive]
Maybe T8s will be rare?