Hi Elmar,
I believe what you have done is fantastic. You must be proud of your code. But the difference between freeware and open source only hurts the users. I mean, you are not making any money from the product right now, nor you would if you open source it. So, it is the same from that point of view.
However, things would change drastically for end user should you ever choose to open source Plop boot manager. Why? Simple, in the worst case scenario: peace of mind. Peace of mind to know that the software wonīt become abandonware once you change jobs, lose interest, donīt have time, move to another country, change profession, or, god forbid, get stuck by lightning while camping. ;-)
In the best case scenario: some other people could take a look at the code, do suggestions, or add their own routines to create new functionality, off-loading some of the work from you. The "creature" would take a life of its own.
You could still manage the central repository, you will still be named on the program source code as the creator, and you īll earn fame as "the guy who created and open sourced Plop".
Well, just an idea.

Wishful thinking, at best.
Would love to know what you think.
Best regards,
FC