I'm not sure where you got the idea that witnesses are supposed to work 100% of the time for Hive, but that's just not the case, and it never has been. Witnesses are voted on primarily to act as trusted block producers, so the main criteria is that stakeholders consider them trustworthy.
It's also expected that the top 20 witnesses will keep informed about proposed changes to the blockchain rules (hardforks) so that they can make reasonable decisions about what changes are best for the network and when it makes sense to deploy such changes. This doesn't necessarily mean they are programmers (many aren't), but they need to have an understanding of what's being done by Hive devs.
It's true that top 20 witness do often make other contributions that help them to get votes, but there's no requirement of that type, and some of the current top 20 are top 20 because of things they've done in the past, rather than now. Again, it's mostly a reputational thing that makes you likely to be a top 20 witness.
The proposal system (DHF) is the primary mechanism for paying for dev work, etc, not witness pay.
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