right now the command autocompletes when curves are preselected, but not so when selecting them manually. EdgeSrf on the other side does autocomplete once 4 edges/curves are selected though that works probably on counts, but since PlanarSrf relies on curves being in a closed loop maybe detecting when the loop is closed could autocomplete the command either?
edit: Would it be possible to implement Chain edge/curves for this command to make difficult to select parts easier?
Problem is one could have several nested loops - i.e. a surface with holes…
yes but these should be closed loops either, so no matter how many curves/loops, when they are closed it should be ok i assume.
Yes, but if you select the first closed loop (say, outer) and the command autocompletes, how do you select the second (inner) one?
thats true, and would not make sense to have this working on preselect only. but maybe a chain edges or even chain edges/curves option and ideally an autocomplete when closed loop option could do.
also this was actually meant for edges only and not for curves, for curves that could work but one can easily select a few planar curves in one go so that should not be difficult, but edges are sometimes a bit tricky to grab and here it would definitely make sense to autocomplete the command.
You’re still not addressing how to pick multiple curve loops there is no way for the command to ever magically know you are “done” until and unless you’ve selected every valid piece of input in the file.
There is nothing to be gained from this pursuit of trying to eliminate every supposedly “unnecessary” Enter press.
i guess you did not pick up what i wrote above so i quoted it once more.
imagine there is a new road built, you are not forced to use it, you can use the old one that goes around the problem and takes you longer, if that road still exists then. are you being sentimental? 
Rhino’s UI is not very good. All requests to “improve” it are worse.
You’re apparently(after having pointed out to you how your initial idea doesn’t work at all, you’re confusing how it works with a totally different tool,) asking for some sort of new different command that’s more like “cap” or something that will only work at all nevermind “better” under extremely specific circumstances.
Selecting the input you want and hitting PlanarSrf exists already and is the superior workflow in all situations where it is possible.
after tediously selecting several edges that could not be mouse drag selected to close certain planar areas i felt the need for a simpler version of it. edgesrf does autocomplete at least. the fact that this can not work here was already dissolved, but if you think its needed to beat a dead horse deader then ok, be my guest 
yes something in that direction, but not closing all planar loops at once of course. i still think that having a reliable chain edges could be the way out here.