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".
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
In sports betting markets it is theoretically possible to make money, but very hard.
https://en.wikipedia.org/wiki/J._Doyne_Farmer#Beating_roulet... (although the roulette example could be classed as cheating ;)