Mike Mearls did say early on that the math is designed with the idea that your character's primary stat will have a +3 bonus the entire game. They've also said that magic items aren't figured into the math. The point of this is that every bonus you get (whether through an item or through an Ability Score increase) actually makes you better (as opposed to just keeping up with the math).
Now, that being said, I played around comparing PC attack bonuses versus monster ACs at various levels. What I determined is that if you start with a PC with a +3 bonus to attack, and assume they receive their share of the number major magic items XGtE assumes you'll get by certain levels, and assume that about half of those items are weapons or armor, and assume that character advances their primary stat bonus up to +4 and +5 at their earliest opportunity...
...the chance for one of those PCs to hit the average AC of a monster of CR equal to their level is 65% at all sampled levels (1st, 5th, 11th, 17th, 20th).
I'm not sure where that discrepancy is coming from, because that seems really convenient if it's an accident. It would, however, mean that Mike Mearls mispoke, or I misremembered or misunderstood him.
That being said, 5e really is easy mode, and regardless of what the numbers I derived say, adding magic items does make your party strong enough that the same threats will be much easier to face.
But I think it is fair to say, regardless of how later advancement works out, both that the game's math assumes a starting character will have a +3 bonus in the primary stat, and that not having a +3 isn't going to be a problem.