Of course, I'm no "pro-trader", but I'm quite sure that "pro-traders" have such a stop condition.
First of all, there are lots of trading strategies - e.g. momentum-trader will definitely use different exit-signs than value investors. Since you wrote about buy-and-hold I assume that we are talking about some kind of value/growth-investors (without defining exactly what strategy makes up such a value/growth-investor), because buy-and-hold only makes you earning money if you buy the right stocks to hold. Therefore, you need some "metric" how you find out which companies you want to buy - in many cases buy-and-hold is closely linked to value since both are interested in stocks' developments over long periods of time.
In my mind, the idea behind buy-and-hold is to buy a stock that is cheap/underrated and hold it until the share price reaches a level that exceeds the "fair value". The calculation/estimation of the fair value depends on the specific strategy of the investor. Of course, it can be a limit like 40$ but often it can depend on more sophisticated numbers from the balance sheet or secondary metrics like P/E ratio and so on. Since a single number can't provide enough information to draw a conclusion about a company's value pro investors definitely combine a couple of information from the balance sheet.
So, to come to your qualified objection that it seems strange to believe that the stock price is going to rise "forever", let's take a simple example.
Assume we have 1979 and the audio-tape first walkman is released. A value investor notices that Company A which produces high-quality walkmen and sells them for a fair price is underrated regarding his metric (so it is significantly below "his" fair value). He decides to buy the stock (e.g. for 10$ per share). In the first months, the stock goes down for 8% but since he is a value-investor he doesn't care about such short-term fluctuations. Then, the stock price rises and falls, such that one year later the stock still is 5% cheaper than he bought it. Maybe the buy-and-hold investor re-checks his calculations and if he still believes in the company he decides to wait for another year. Suppose now the stock rises and in 1985 and Company A is the market leader and its price is 25$.
But technology has changed - meanwhile, the first discmen have been released by other companies but Company A missed to develop such a product. The tape-based walkmen's popularity has exceeded it's top since a new, better technology was developed. But Company A's success heavily relies on tape-based walkmen. So even for a buy-and-hold investor, there is no reason to believe that Company A's stock price will rise even more. Contrary, he worries that the price will drop significantly so he probably will sell his stocks.
Therefore, the main attribute for buy-and-hold investors is that they don't care about short-term fluctuation in share price (in the time-scale of days, weeks or even months) but they do care about changes over years and of course they do care about future prospects of the companies they hold. So they definatelly have a stop condition, but I would not expect this to be a bare number like "50$".