Agree 100% with you that "only something that doesn't cost them anything would work" in terms of increasing adoption.
We've talked to many companies, workers, founders, CEOs, etc., and while interest in crypto, owning crypto is generally high, not many people are willing to take their entire compensation for it (a la Bitwage). This is why we've built a more passive way to receive for employees - more as a perk, a reward, similar to how Lolli makes BTC a reward for shopping. And so all the companies we work with today treat BTC as a perk alternative that an employee can choose if they prefer it over, for example, movie tickets or gift certificates.
Bitwage actually lets you take as much % as you want as Bitcoin, and I know now also there are some solutions like BitPanda (I'm not 100% sure how it works so I could be wrong) that let you also set your "savings" automatically converting either a set amount or a set percentage of your balance in fiat at a set time. Sort of like enforced savings systems that I manually do myself anyway (except the other way round, converting a forced amount of btc into fiat!).
But both still mean you actually lose money (via exchange rate and commission) which is why there's still poor uptake IMO. Good luck and let us know how it goes.