Have the MDA20 Dual Monitor Arms/Mount, now only need Single Arm (MSA20). Same arms, different base.

I previously owned two u2720qa 27" Dell Monitors that I used the MDA20 Dual Monitor Arms/Mount. I recently replaced those two 27" monitors with the Dell u4025qw 40" 5K monitor. I want to mount this monitor to my desk, like I did with the dual 27" monitors. Looking at the MSA20 (Single Monitor Arm/Mount) and the MDA (Dual Monitor Arms/Mount) they're identical in every way. The only difference is the mounting base/plate. I spent almost $300 for this MDA20 and would like to repurpose it for my almost $2K Dell Monitor that I purchased. I don't think I should have to pay another $150 to get the MSA20 when I have the MDA20 with two perfectly functioning Monitor Arms (only need on, obviously). I just want to purchase the MSA20 Base to attach one of these arms to. 

I reached out to Support Chat, but that was about as helpful as asking my 9-year-old son. He said that I would have to buy the $150+ MSA20. Pretty frustrating. If that's the case, I'd rather just get the Ergotron from Amazon and be done with Dell's mount(s). 

Is there anyone I can reach out to? I mean, why would Dell not sell replacement parts? What if I owned the MSA20 and the base cracked or something. Would I have to rebuy the entire thing, or would I be able to get the replacement part only? 

Any help would be appreciated!