people, are you really know what are you talking about ?
yabause - afaik there was no side contribution, in fact project seems almost dead for a years, even it OS and far from finished and have really bad (compared to SSF, closed source, made by single person) compatibility.
retroarch - made by very few people, not sure there was anyone's other code in the core.
MAME - VERY HUGE project, there quite many pull requests, but from people which is lets say "almost" the MAMEdev and close to team. but almost ZERO contributions from really side persons.
Final Burn - no side contributions afaik. (except from several from a guy who port it to Xbox)
Desmume - no side contributions
GlideN64 - this project OS is more than a decade, but all this time was improved and maintained by SINGLE person (Gonetz).
NullDC - no side contributions at all.
reicast - there was few interface/backend/portability side code. but no any emulation related. with iirc one exception - DC microphone emulation
lxdream - no contributions. project dead for a long time.
bsnes - no contributions. all was made by byuu only.
and I can continue and continue and continue this list of open source emulators to very very huge size.
but results will be the same - there no any benefits for emulation project going open source.
with a very few exceptions like Dolphin is.
Desmume has no side cotributions due to using SVN and Sourceforge.
Same with Yabause but now after switching to Git and Github it finally got a new skilled dev who is improving hardware rendering. Also with Saturn there is the lack of documentation and Saturn emulation never really got any open source traction everyone was happy using closed source SSF.
RetroArch did get many side contributions although major parts were written by a few people,
NulllDC has a very bad codebase used googlecode now as Reicast it is using github but code is just as bad as before it's heavily focused on Android and devs don't seem very interested in improving emulation due to performance reasons. Moreover many people were content with simply using closed source but superior Demul instead of contributing to NullDC.
Bsnes did get some contributions from others mostly from people on byuu forums but not much due to lack of public repository or any version control system same with mednafen. That was deliberate choice of these devs.
With Glide Orkin(original dev) didn't use any version control system or a public repository and codebase was a mess. Gonetz too for a long time was working on it on his own in private without public repository and later interest in N64 emulation died out though now on Github N64 emulation is again getting some activity.
You definitely should consider making Demul open source. Even now as closed source one man project it's still the best Dreamcast emulator so far. Even though competition isn't very strong it's quite an achievement but by making it open source you give it a chance to reach Dolphin's or even higher level.
Of course opening a source and leaving it at that is not enough to achieve this. It took Dolphin's many years to reach that level and good management while those projects listed by you were mismanaged. To attract new developers you probably would have to do things like progress reports, technical docs about Dreamcast's architecture, emulator infrastucture with hardware tests, public forum and the codebase would have to be readable for outsiders. A lot of work but could be fruitful.
After PCSX2 switched to Github it took like half a year before they started getting new contributors.