I donāt think āincentivesā would help. In my eyes testing is the hardest part of the development process as you donāt have clear defined goals like āit compiles letās sell itā ( only joke 
). working through well designed runbook and carefully looking for issues which might be not obviousā¦ especially client testing is really hard - running this in production is bad idea, finding good test coverage with production like usage (donāt forget all the moving parts like client reboots, hibernation, network outage, software updates, interaction with other software packages like anti-virus, different OSā¦) is a nightmare - I donāt think you can count on this forum to find highly trained people + willing to spend big amount of their spare time + to test beta softwareā¦
If you want good results - testing must be done by the most experienced people you have - this is the reason majority of testing should happen close to the devs - they are expected to be familiar with the product, know where to look in case of issues and can involve their colleagues fastā¦ This forum can help when it comes to real-life testing on different platforms and cover all the variations of real-world installationsā¦ but this must be the very last testing stage with solid, well tested versions you already consider production ready but need last confirmation.
Iām the last one who blaims you for this specific bug - I know the competing market, demanding customers - but I think there is huge room for improvement in communications between Nextcloud GmbH and the community as discussed in depth here