I disagree, paying customers wouldn't put up with a malware infested version of FileZilla, this stuff only really exists at the "free" end of the market.
This is empirically false. Video games have shipped with highly-intrusive rootkits and malware disguised as DRM for years, and it tends to be worse on the higher-end products, vs the shovelware/free-to-play/open-source.
You can find a few counter examples but they never last because the commercial pressure is too high, the obvious example is Sony, are they still deploying rootkits? Lenovo is another example that has started cleaning up its act.
Would these companies change if their only source of funds was the malware? I don't think so.
Not really, It's just that it's obvious at the free end of the market.
Proprietary software can do whatever it feels like on your computer and you would be hard pressed to know until it was too late.
A few large companies have been implicated in root-kits / backdoors / random horrible deliberate security practices. These are probably just as destructive as replacing your browser search bar or installing some fake AV software.
Free isn't the problem. Bundling crap-ware with otherwise audit-able open source software is the problem.
