Two words. Service agreements.

Many of the other DCI carriers still operate under "cost plus" contracts, which means a profit margin and performance bonus/penalties tied to the cost of the operation as negotiated. There are disclaimers that allow costs to increase inline with other carriers. The bonus method is paid by delta and not tied into "payroll" to be used as a cost basis of the operation, meaning the other DCI carriers can't base their costs on the added pay, as it's a "retention payment".

So, to summarize, DL can offer Endeavor the retention payments, which pilots see as direct compensation, but it keeps DL from having to pay any more than the already contracted rates to other DCI carriers. If you look at the service agreement timelines for renewal and expiration, and Endeavor's JCBA (pilot contract), the bonus timeline falls right inline with all the other timelines. So, in theory, the future pay rates will increase instead of a bonus, but will be done when there is no impact to other carriers or overall costing when the next round of contracts, both pilot and service agreement(s), are signed.

That's how I see it all at least.