My import work has never been busier
You see, that's one of the problems though. Everyone who migrates to XenForo needs an import, so unless you are starting a new forum, 100% of people need a migration.
That's not the same for the add-on market. Plenty people don't need add-ons at all, or only a handful, and only very specific ones. If you want to survive from broad market addon sales, you need a very big portfolio to fill enough needs that people buy some of your products. As you stock up your portfolio, maintenance work will quickly outweight any new sales, so unless you'd drive an aggressive no-support-unless-explicitly-paid policy, you'd probably just drown in support work at some point. And many products you develop may never make back their time investment in the first time, given how hard it is to gauge market interest. This is additionally made even harder by features being taken from the add-on market and being integrated into the core software, effectively blowing up income streams of high selling products from one day to the next.
If you do individual client work, then you're also constraint to the people that have the budget, which is only a fraction of the community, unless you intend to sell yourself under value just to have some bread on the table in order to compete with developers from countries that have lower living expenses. A lot of people would just outright decline if I named them the hourly rate I had to charge to support my (pretty average) lifestyle, and that doesn't even include the extra budget that is needed to fuel an agency but just a single person.
There's of course a lot of bigger clients in the market, and we happily work with them every day, but when they don't need some of our add-ons, it's just not viable to keep them around anymore. And it hasn't been for years.