sub-minute opportunities are absolutely not all that exist, in many ways it’s the opposite. The hft opportunity space is extremely competitive, and much more zero sum than the mid/low frequency space. A relatively small set of players compete for the lions share of anything one might consider high frequency. The set of trades you compete for and the relevant information to generate forecasts is limited / similar across firms, and table-stakes on engineering side are very high.

> If there were long term strategies that were reliably profitable, do you think they would simply ignore them?

Yes? Mid frequency trading (say holding period of 5 minutes to ~1 hour) is a totally different class of quantitative trading, from the features you use to how you build portfolios (if at all) to how you execute on forecasts. At least one top-of-the-game hft firm (that already relied on forecasts instead of speed) got burned on their first attempt to enter mid frequency.