Doing all these would allow you to surface better results than using just one. But it comes at a cost.
Yandex is Russian and has pretty poor results. And you’d be naive to think that those of us concerned about privacy would ever touch something built on top of it.
In other words there aren’t paid search engines that DuckDuckGo could turn to. Unless they build their own crawler, the only game in town is Bing.
And yes you can compete with Google (for a certain target group) by paying them for their search results. Results are just a distribution channel, it is what you do with them that matters. For example Google and DuckDuckGo both choose to show you ads and affiliate links but that is hardly the only option.
Sorry, but I don't trust DuckDuckGo that much and when technology fails, I'd rather use a search engine based in a country that has rule of law ;-)
They're desperate to collect user click data because they know that's the only way they'll have any chance of success. Even anonymized, that's very valuable data.