Yes, they can be.
I have launched and run two successful boxes having sold the first one for a 2x profit on my share of the business. This was also 2012 when the supply hadn't quite reached the demand though.
With CrossFit having reached the mainstream and most major metropolises having reached a point of saturation it's literally no different than any other startup. My current gym is successful by most owners standards with 200+ members. After 2 years of no wages I am now able to pay myself about $60k. Next year will be better as we will need less reinvestment.
I would say the days of opening an average gym, with average equipment, staffing and location is a sure fire way to be overworked and underpaid. If you like being able to go home at the end of the day and not way about paying the rent on the building then you need to having a solid business plan with intent to fully execute. Otherwise I wouldn't quit your day job.
Over the last year, three very average locations closed up around us. Every one of the owners, including the one that now trains at my gym sites having to coach every class, every day and only making a couple thousand dollars after overhead is paid as the reasons for him closing up.
I do think 20% of the gyms are making 80% of the money in the space. I also believe you have to find a way to seperate yourself. If you can pull it off, it's a great way to make a good income doing what you love.