There's no excuse for not making search engine management as simple and flexible as Firefox does, especially on the desktop, but I appreciate the hoops you jump through to keep hacks available for browsers with less user choice.
No doubt it is revenue-related, but it's also a privacy problem.
Can you describe how? What information is that query parameter providing to an attacker that is not being provided in 50 other ways?
(My impression is that the feature is hidden behind a setting because it's half-baked and not yet ready to replace opensearchdescription xml, add-ons, and bookmark keywords as the official way to add search engines.)
For those with tech knowledge who want to try to change things, I recommend checking out https://www.techcongress.io/, https://horizonpublicservice.org/ (they both have a cohort they're recruiting for right now) and USDS.
That said, keyword searches are awesome and I use them for a whole lot of things, including effectively custom search engines. I have keyword searches against, e.g., internal confluence docs so I only have to enter "con <thing I'm searching for>" and firefox will load the confluence search page, or "people <name>" to bring up our internal phonebook with a search for the employee. I don't think I've ever needed to add my own entirely custom search engine to the browser.
The regular way is to visit the homepage for the search engine you want to add and click the URL bar. This will show the usual search drop-down with the "This time, search with:" row at the bottom. If you're on the homepage for a search engine which hasn't been added to Firefox, there will be an additional button on that row allowing you to add the site as a search engine, which will make it accessible from the regular settings page like every other search engine.
It doesn't.