This feels like trying to win the lottery? Why not go find keyword domains that are missed or undervalued in other extensions? Obviously the keyword matters and it needs to be taken in multiple extensions but a 500 dollar flip now feels more realistic than maybe someone 10 years from now will pay me 2,000 for something I made up. Thoughts?
TLDs besides COM generally sell less frequently and for less money. If you are a beginner you aren't going to be maintaining a portfolio of hundreds or thousands of names. If all you have are 10-20 names and they are all in some no-name TLDs, you might go for many years without selling any, and then you may feel discouraged or encounter financial difficulties at some point and let the names drop, taking a loss, never really sure if any of your names were any good. A beginner really needs a sale at some point, to offset their initial cost and give them motivation to keep going, and COMs give them the best chance at making a sale.
Note that almost all TLDs cost around $10 and up to renew, and COMs are near the low end of that scale. So you pay the same or lower renewal fee to maintain a COM name as any other, but it is likely to sell more quickly and for more money. In your example suppose it takes 5 years to sell the COM but 10 years to sell the non-COM. You have to pay $50 to eventually sell the COM at $2,000 (40x profit), while with the non-COM you are paying $100 to eventually sell it for $500 (5x profit). The profit margins are much less for the non-COMs.
Also consider that beginners are not very experienced at picking names, so the names they pick already have a reduced chance of selling, which means on average they are paying for more renewals before a name sells. So what if (from your example) you are wrong (being a beginner) about a name being good just because it is registered in a lot of TLDs, or the name is basically not really compatible with the few TLDs it is not registered in? When you factor that in, the beginner may end up losing money on those non-COMs. Perhaps they will only ever sell one of the five names for $500, and it takes 10 years, but in the meantime they have paid $500 in renewal fees.
Essentially the reasonable renewal fee, the faster time to make a sale, and the increased price at time of sale all help a beginner to reduce the cost the mistakes they are almost sure to make.
Even a stupid name that a beginner might choose, like "gamesz" (games with a random consonant at the end) might have a chance of selling in COM but probably would not ever sell in another TLD. The idea there being that if anyone wants the name for more than the cost of a $10 hand-reg, it's only really going to be one person/entity, and they are probably going to want the COM. Because what are the odds they are willing to accept a stupid name in a stupid TLD (two stupids) for their website? They may just try to find another name in that case. If you bought the COM then you might luck out and escape with a profit, but if you bought something else you're just paying to maintain trash that will almost definitely never sell.
The one exception is if you are hand-regging TLDs like XYZ that cost $1 or so for the initial registration. In that case there's probably no reason not to go to town and register anything that seems decent, as long as your plan is to only hold it for a year and not renew. You get 1 year to sell the name and if not then you drop it and all you lost is $1.