You are correct that using a forward PE is speculation, which is...

You are correct that using a forward PE is speculation, which is why people do their research, follow the companies development, the sector it operates in and the prices it receives for its product. However, most if not all, use a forward PE to assess the prospects of a company, which informs whether to buy, hold or sell the stock. Therefore, said research removes about 70% of the speculation.

Using or relying on a trailing PE to inform your buy, hold or sell decision is both lazy and high risk, in that lack of forward looking research would mean you miss gains or incur losses. If that is what you do before you invest, you are open to disappointment, particularly commodity/mining companies, as commodity equities follows the price of the product they are producing, both up and down, commodities 101.

As an example, if a mining companies product pricing was high due to cyclical supply/demand imbalances, and therefore their 2021 financials were very strong, the 2021 trailing PE would be a buy signal for you, and without any research into the forward looking near term supply/demand fundamentals, you could be buying in at the top of the commodity price cycle. Nonetheless, you were speculating that the strong 2021 financials would continue into 2022 year.

So, probably best to leave your money in a bank account, yes a bank account not a term deposit, as interest rates are rising, and terms deposits are likely to pay better interest 12 months from now, but the bet, interest rates will be higher is speculating, isn't it....