The reason proficiency starts at +2 is so 16 mod(+3)+2 = +5 or hits AC 13, the standard CR 1/8-CR 3 AC, on an 8. Or 65% of the time, essentially 2/3rds of the time. Which is basically what 4e does.
Just in general, 5e uses a ton of 4e/2 math. You can expect to roughly go up +9 to hit over 19 levels instead of +18, +6 to important skills instead of +12, get approximately half the magic items of an inherent bonus 4e game, etc...you just don't have quite as much clarity to the specifics of when you'll get some additional bonuses or the equivalent to hit in 5e.
I would expect that math to continue to make D&DOne usable with 5e adventures.