Just my two cents...
I've been offering conversions to my customers for the past 4 years, and have done quite a few. Here are the following situations where a conversion might make sense:
1. Recumbent trike
2. Adult upright trike
3. Seeking custom high power and larger battery
4. Very low step bike, where OEM's aren't serving market well,and you can get something like a biria easy boarding converted.
5. A bike you just absolutely love as long as it's not real specialized such as a triathlon bike, only a few years old, or doesn't have non standard components.
Otherwise very good ebikes are on the market today at price points that make it hard to beat pursuing a conversion. If you can DIY it, then a conversion may make sense, but if you are doing it to possibly save money, something like an Aventon Pace will be a better buy than most conversions done by a third party.
2 or 3 years ago, this may not have been the case, but the market continues evolving rapidly and doing well despite tariffs.
P.s. doing one on an 'old' bike is generally not a prudent idea. Wear and tear, hidden corrosion, older (possibly obsolete) components, a bike that maybe has a bluebook value of $75, and adding $1000 of new e assist, generally leads to something you least expect suddenly breaking, and making the conversion a continuing sink hole for repair money.