It's not actually necessary to use reusable rockets to get at a similar cost per launch. Long March 5 is at 2.8M$/ton (so less than SpaceX commercial price).

Also, 15M$/launch is not widely believed to be correct. There is much creative accounting SpaceX could be doing with Starlink (is at-cost account for booster depreciation? If so how, since we don't know how much reuse a booster can be expected to give? Or is it just the cost of refurbishment?), and since the last statement where Elon claimed 1000$ per kg actual cost, SpaceX had to raise their prices, claiming it was due to inflation - is that accurate? Most estimates I've seen are that the cost is 20-30M/launch, which would instead give 1.1-1.7M/ton.

So, it's a big advantage but not an insurmountable one.

Boosters don’t depreciate. They are actually considered more valuable with more successful launches.