I don't think the fact that your coin-flip martingale strategy didn't work is evidence that "there aren't any betting strategies that can last over the long run".
No, but over the long run, no betting system can guarantee a positive return. If you run enough simulations, you will inevitably lose. The Wizard of Odds[1] did a great piece on this and it's definitely worth a read.
Also, if you believe to have a betting system that does work, VegasClick[2] runs a $30k wager against your $3k if your system can survive at least 11 of 20 200,000 round games. It's easy money.
I think the other posters mean sports betting and other betting markets when they say "betting" not just casino games where, as you rightly say, there's no way of making money.
In sports betting markets it is theoretically possible to make money, but very hard.
Also, if you believe to have a betting system that does work, VegasClick[2] runs a $30k wager against your $3k if your system can survive at least 11 of 20 200,000 round games. It's easy money.
[1]: https://wizardofodds.com/gambling/betting-systems/ [2]: http://vegasclick.com/gambling/betting-system-challenge