This is just my personal opinion but I have a hard time seeing how a SWUs/instrument/etc. would create this -- especially if the expectations were managed correctly.

CO already has a way to control upgrades to BF (via the upgrade bucket) and has allowed mileage upgrades for the past (two?) decades. Likewise the OnePass base - and arguably the top-tier more so than the rest - are already familiar with the upgrade schema, clearings and co-pays.

For example, if CO allowed customers to get around paying the mileage requirement (the copay would still apply) for a BF upgrade they'd still be operating under the existing system of upgrades (which customers, especially long-haul the elites) already know, they'd still be managing the revenue from the front cabin (by rev-management controlling the upgrade bucket), they'd still get the cash from the copays, any fare could be upgraded (there's no minimum fare requirement, just like it works now) and they'd be more in "alignment" with their other *A partners (AC/LH/UA.)

That said, I'm just a FT'er and suspect there's a few compelling reasons we just don't know or understand that would quickly whittle away the simplicity (and usefulness) of such an idea. Oh well, c'est la vie.