Very good point, but sometimes trends will continue. For example, the price of oil has been declining for a few months, and is continuing to follow the trend of declining. Investing in oil right now for short term would not be a good decision. Of course, if you have money to spare, you could invest in wait for the price to rise, but if you need the money in the short term it wouldn't be advisable.
Unlike price movements in a market, the game results generated by MoneyPot are statistically independent. So while trends can certainly (randomly) appear, they are not indicative of future results. And since MoneyPot only accepts wagers with a positive expected value for the house, the chance of a downtrend breaking should be higher than the chance of a downtrend continuing.
Very true, however I'd say that in my opinion, MoneyPot is on one of those random trends, as it has been declining for a while now.
Also, MoneyPot does have a variable house edge, so perhaps at the moment people are using the bonus cashout strategy and it's working for them.
However, I think you do have a good point, like you said, the house edge will get them eventually, so investing is always a good option.