Arguably, if someone managed to convince the CEMU team to open-source it, open-source WiiU emulation would be in a better position than if all of those volunteers started working on Decaf right now. CEMU has managed to do a chunk of reverse engineering (or reading stolen Nintendo design documents that they do not have the legal right to have) that Decaf would have to duplicate.
Part of me is fearful that the reason they won't open source is because they stole design documents. Anyway, people have tried to get them to open source countless times in the past, they won't budge.
If I remember correctly, they're considering that option for the future, but not anytime soon.
Didn't Dolphin start out as closed source, then go open source after a bit?
I hope they open source it if the original creator loses the drive to work on it, though.
They're currently making nearly $200,000 a year on Patreon, so it's also possible that they think letting other people contribute would undermine the legitimacy of that. If I pick out OpenMW (and its fork, TES3MP) as an arbitrary open-source project, it looks like between multiple people, $2,000 a year is made via Patreon as it's harder to incentivise people to pay money for an open-source project as you can't offer rewards like Patreon-exclusive builds without violating the GPL. OpenMW has half as many stars and people watching on GitHub as Dolphin, and I reckon CEMU isn't ridiculously popular compared to Dolphin, so if people were as willing to pay for open-source development, you wouldn't expect the factor between CEMU and OpenMW to be 100x.