In my mind the Sessile Question can be divided into mutiple subquestions which might be easier to answer seperately.
A. Should sessile organisms be autoevolved or static props?
I think making them static would go against very fundamental tenets of Thrives design philosophy and as others have mentioned, it wouldn’t even necessarily be less work intensive. Making autoevo simpler and more streamlined for sessiles like hhyyrylainen mentioned is a very viable possible compromise.
B. Should the player be able to play as a sessile species?
Mostly yes as barring the player from playing certain kinds of species would also contradict fundamental missions Thrive set out to do. I also imagine “sessile” to be a hard thing to properly define. We can’t stop the player from building a body plan which doesn’t lend itself to locomotion. Would we stop the player from leaving the editor if his organism can’t move?
An anemone moves one centimeter per hour. Is it considered sessile? Where do we draw the line?
C. Should we be expected to provide players who choose to play sessile species with engaging gameplay?
This question I believe can more readily be answered with “no” than the other two questions. A game can never be equally enjoyable in all of its parts and while it’s on the developer to give the game fun parts, it’s in the agency of the player to seek out the fun parts.
I guess the minimum we could do to make sessile gameplay less unplayable is giving players the option to speed up time when their movements are below a certain speed.
I’m unfamiliar with how difficult it is to implement such a time lapse system, but at least I know of a few games which have implemented it. Kerbal Space Program and Kenshi come to mind.
Nevertheless I’m sure that would be difficult to programm and even this minimum of making sessile gameplay less jarring isn’t a must imo but more of a nice to have.