While it is true that nearly all dot com domain names are brands, this does not mean that all brands must be dot com domain names. I would not advise against choosing a dot com domain name for a brand, but there is very little sense in registering a name like verylongandhardtoremember dot com.
There are perhaps a handful – at most a few dozen — domain names which describe something like „marketing categories“ of information. You might consider such categories to be roughly equivalent to magazine titles (e.g. „People“) or something like traditional publishing ressorts („sports“, „travel“, „business“, etc.). There are indeed also names registered in dot com for marketing categories which might be considered to be niche markets (e.g. „movies“, „cars“, „hotels“, etc.)… and quite often the websites for the corresponding domain names are search engines for that category. These are, however, more often the exception than they are the rule… especially in the dot com registry. The reason why this is so is that over the past several decades people have become accustomed to the idea that the dot com top level domain (TLD) is intended for brand names (e.g. it is quite plasible that no one should expect amazon dot com should be anything other than a commercial entity). Ironically, this seems so blatantly obvious that it almost seems odd when someone registers their personal name as a dot com address without it being the website for some sort of commercial enterprise. Dot com has become the quintessential „fictitious business name“ registry – there is no need to create a dot dba registry (for „doing business as“), as this is de facto what dot com has come to mean.
So while hotels dot com may help someone to find a hotel, it is first and foremost a commercial entity. Just as not every person is to be found in People (Magazine), so too not every hotel will be listed in the hotels dot com „lookup“ directory. Indeed: Just as you may find out more about some people in magazines other than People (Magazine), you may also find out more about some hotels at websites other than hotels dot com (and you will probably find out next to nothing about the Amazon region at amazon dot com).
Dot com now has so many registered names that well over 99 out of 100 dot com domain names are not marketing categories (let alone English words). In other words: More than 99% of dot com domain names are brand names.