It would be great if you could get a 25% - 30% conversion rate on anything! It seems the commonly accepted figure is in the low single digits for conversions on any product - largely irrespective of market sector. So if you’re selling something online you should probably assume that on average only 5% of your traffic at best will convert to paying customers. In some cases 2% might be closer to the truth.
If this happens at each significant decision process in the ‘funnel’ then you can see how you go from ‘the world’ to ‘a very few people’ very fast. The biggest mistake people make (and I’ve seen it happen) is the assumption that everyone in their target market will become a customer.
In terms of donationware (and I released my software as donationware 15 years or more ago when I first published a few Linux JACK ‘plugins’) my experience was that the software was well liked, but approx 50% of people didn’t donate because they thought nobody else had (so why should they), and the other 50% didn’t donate because they assumed everyone else had (so they didn’t need to) 
But - there is actually a ‘behavioural engineering’ element to successful donationware, intentionally or not.
For example, if you publish a finished work, that’s feature and functionally complete it seems more difficult to entice people to donate (they don’t need to support future development it completely works).
If you publish something that’s completely broken or unusable, people tend not to donate because its just not very good. More effective it seems, is to create something which works ‘just well enough’ - that it has potential and is tantalisingly usable, but would be better for having the support of donations (and future development) ‘Its nearly finished’.
Equally, if you ask for donations via a paypal button and an anonymous download, while most are decent people and would like to do the right thing, if its an anonymous process, its too tempting to just take the free download (no-one ‘sees’ you do it) and “maybe I’ll donate something later…” - which, with the best of intentions tends not to happen (some distraction occurs or whatever).
Conversely if you ask for something identifiable - even if you never use or store it - perhaps you say ‘please sign up with your email address so we can send you the download link’ there’s a subconscious trigger that ‘someone somewhere might know I didn’t pay’ - even though that’s completely permitted -and it can act as that uncomfortable phrase “a nudge” to donate. Think of it like the virtual equivalent of the ‘honesty box’ I guess. What I’m getting to is that how you set up the system is absolutely key to whether it’s successful or not.
If you got this far, thanks for reading my totally amateur thesis on SEO (it is however based on some personal experience over the last 15 years of trying to run an online business - and yes I did naively assume that 100% conversion rate was of course completely feasible - all you need is a good product, right? )