Strategies come and go. Sometimes value investing is knocking it out of the park while growth or momentum is lagging, or vice versa. You are correct that a strategy won't outperform every year. Eventually there is performance chasing and the strategy will mean revert.
Look at managed futures funds in 2008. Some of them were up 20, 50, 70%+ while the rest of the market was down by 30%+. It was a banner year for these funds, better than they've had since probably the early '90s. A ton of money piled into these funds and they've sucked ever since.
A lot of value investors had a fantastic year last year. It is most likely that they will underperform for some time in the future.
The pros understand that the strategies that have historically outperformed over a long time, but have sucked the past 3-5 years, are the ones to look at. The problem is that the individual investors are always chasing performance. They chase the funds that have done the best, when they should really be looking at the ones that have done the worst. Obviously, you need to weed out the funds that suck because their fees are high or their strategy is poor.
All that said, most individual investors will do best with index funds and something like a 50/50 or 60/40 portfolio, diversified throughout the world and not just a US-focused allocation.