Yes… and no
To clarify my seemingly contradictory statement, I’d just like to add that another question is (in my opinion) necessary in order to answer the one in the title:
Which dot coms?
You see, there’s no way to just say “dot coms” in a manner that encompasses absolutely everything in terms of inventory.
For example, Games.com and FunNewJerseyGames.com… do they both fall into the “dot coms” category?
I’m sure you get the point.
Let’s not generalize.
The answer to a simple question is unfortunately often complex and our situation doesn’t represent an exception.
For example, I’m reasonably confident that Insurance.com will always (unless something dramatic happens) be the category killer domain.
Its value will most likely not be affected by new gTLDs in a negative manner and in fact, the exact opposite is more likely: the effect of new gTLDs on the very best dot coms such as Insurance.com will most likely be positive.
But what about let’s say BestInsurance.com?
It’s a great domain, no doubt, but what if you were to choose between it and Insurance.web for example?
Some people would probably choose BestInsurance.com, some people would probably choose Insurance.web.
Therefore, it’s relatively safe to say that under such circumstances, the value of domains such as BestInsurance.com might be affected in a negative manner to a certain degree by new gTLDs.
What about let’s say YourBestInsurance.com vs. Insurance.web? In my opinion, most people (the overwhelming majority) would choose the dot web.
Therefore, it’s relatively safe to say that under such circumstances, the value of domains such as YourBestInsurance.com most likely will be affected by new gTLDs in a negative manner.
In my opinion, a lot of dot coms will be affected by new gTLDs in a negative manner, as explained through this post.
I’d recommend taking a close look at your portfolio, being honest with yourself and trying to divide your portfolio into 3 groups:
1) gems about which you don’t have to worry
2) good domains which might be affected in a negative manner
3) medium (or worse) domains which most likely will be affected in a negative manner
It probably won’t be all that much fun, being realistic rarely is unfortunately