Make Windows and Mac apps non-free
To motivate the developer and because I personally, as a developer myself, would find it acceptable it would be a good idea to set a fee for these apps.
I don't get free apps. As an indie developer you should be paid for your hard work. I can imagine that, if you receive funds for it, you be willing to release more frequent updates. When you don't get paid the focus will be more on the iOS/Android apps which are paid.
Gabriel Fuertes commented
Well, an outdated android version is also free, and this allow people start using a password manager without paying a penny. Maybe a better idea could be add a donation system like when you download Ubuntu.