Absolutely. Everything that is publicly available knowledge is priced into equities. The only way you could know what the price of a stock should be better than the market does is if you have more information than the market (i.e. inside info).
Buy/hold/rebalance is alive and well.
Large asset managers are financially motivated to sell the belief that the market is about to skyrocket, or plummet. Every time you move money around they get a piece of it (via transaction fees) - whether you were right or wrong.
I'm assuming you mean the US stock market. I don't know (and almost no one does) whether it's over valued or not. I agree that the current valuations (price/earnings) are on the higher end of the historical spectrum. They could go up or down from here though. Also - you have several more options than just US stock market or cash. You can invest in Developed International markets, emerging markets, US bond markets, international bond markets, real estate, etc. All of these have higher expected returns than cash. You can invest in those if you don't want to invest in the US stock market.
*I'm not advocating moving the money from one market to another constantly, it's best to pick an allocation and stick with it.