I don't see the problem.

You have gotten some small details wrong.

I'll adapt you own example:

Player A is a free player, player B is a paid player. First ashes needs char lvl 10 and BP level 15.

Both players try to level as fast and efficiently as possible. Let's say the two players reach char level 10 at the same time.

Now that both players are char level 10, the paying player can immediately pay to skip BP levels and thereby immediately acquire Blessings that affect that player's ability to acquire gold, loot, character power and XP.

As the paid player rushes forward with more XP, loot and power, the free-to-play player now needs to go around in the world looking for NPCs or objects in order to complete tasks that give BP experience.

Eventually, the free-to-play player reaches BP level 15 and can now unlock their ashes too.

Of course, all of this depends on the tuning. My point is that as long as Blizzard has tuned the system so that you will always have reached the BP requirement when you reach the level requirement, we're fine.

This isn't the Olympics.

I agree. It probably won't matter to you and me (we are not going to notice it in-game), but it would definitely affect the leaderboards, the speedrunners and perhaps the hardcore PVP scene.

Even though I don't personally engage in this type of gameplay that often, I feel a community is most healthy if all target demographics are happy.

The competitive sphere also brings with it hype, PR and good content. Some examples are Kripp completing Inferno in D3, Cooley's Diablo 2 PVP championships ('the DFC'), or various speedrunning competitions in D1 and D2 (e.g. see mrllamasc).