If it were there would be no point emphasizing this. It is by no means obvious. Until you experience the two types of locomotives running on the same layout it seems obvious it should work fine. Initially, it does seem to work fine. Then it may dawn on you that it only seems to work.
Motor only dual mode decoders still don’t run a locomotive as slowly as the better DC locomotives run on straight DC power at the same low voltages. The decoder equipped locomotive needs more track voltage before it begins to creep forward than does a well made DC only locomotive.
If you’re just getting into this hobby and at the point of deciding whether to commit to DCC, in my opinion you should just go DCC. If you like some of the older DC locomotives try to restrict your purchases to ones that are relatively easy to fit decoders into.