Uh, that exactly describes a sizeable number of people in my profession (not wrestling or cartooning). Why do you think it’s ridiculous? A person’s salary isn’t the sole determiner of net worth; how much of it they invest is important, too.
A $2 million net worth on a $100K/year salary after 25 years or so is quite doable (especially over the LAST 25 years) by someone with financial acumen and maybe a little bit of luck.
In this case, professional wrestlers have a laundry list of expenses to pay for. Travel, car rentals, food (eating on the road), airfare, the complexity of taxes paid in a gazillion different places. Plus they have to pay for very expensive insurance and a lot of medical expenses. During periods of injury, they don’t make as much unless they’re working for WWE for a downside guarantee, which is a minority of professional wrestler and they tend to make more working for WWE.
Someone making $100k a year on the Indie circuit, then paying those expenses and taxes, isn’t like you or I making $100k. They also don’t tend to have 25 year careers with consistent earnings. Hell, most of those guys have second jobs.
Back to cartoonists, the guy who did Cow and Boy was syndicated for 6 years, then dropped from syndication. He tried to self-publish for a year but it didn’t work out.
As I recall, he also had a regular job during that year of self-publication.
When I was offered a contract to a develop a strip with the Washington Post Writers Group syndicate it was suggested that I might not want to quit my day job. My understanding was that newspapers pay about $10/ week for a new strip and half of that goes to the syndicate. (A rate that hasn’t increased since the 70s.)
Can’t say for sure 'cuz ultimately (after 18 months) my strip wasn’t “optioned.” Boo hoo
Let’s Go Exploring ( 9 minute show segment) just aired on CBC radio this morning. It seems that there was always some tension between Watterson and his syndicate.
Based on the few times that Watterson talked publicly about such things (including in one of the C&H collection books), I think it’s safe to say that that’s an understatement. 
FWIW Charles Schulz is listed as 3rd highest paid dead celebrity with $38 million in 2017 by “Forbes” . The “Peanuts” comic strip has for 50 years been heavily merchandised with tv specials, movies, plays, clothing etc which “Calvin and Hobbes” isn’t.
Bettie Page, who was obscure for decades, is 13th on the “Forbes” list