This is true, there are a few online & offline DXF converters. HOWEVER, most of them don't actually work that well, some screw with the scale, and many of them don't even output a proper DXF file, which is required when importing into CAD/CAM software such as Siemen's NX suite, and especially critical when loading directly onto a CNC machine. Adobe Illustrator excels at this: it's a single step process to import/export DXF files, there are no additional conversion/import processes that can mess up scales, layers, and add in-explainable flaws... what you see in the editor is what you're going to get 98% of the time. When a conversion is not handled natively, such as when you export a file, re-import into an external converter, then export to a DXF file... well, in addition to the additional time required, you explained the issues with that well in your second post, which follows:
And you're right; exporting to a non-native file format and expecting the results to be accurate when opened in a different program is foolish, especially for the purposes of proof-reading. However, there are plenty circumstances where it would be nice to be able to do that, even knowing the results may not be perfect, at least for a while. And no, don't know if you've tried exporting as PDF and importing in the AI, but the results are so bad on any moderately complex vector design, it's not even worth considering that as a valid option.
I could go on, but anyhoo we all get the gist, .AI export would be a great nice to have. Exporting to DXF... well, there really is no reliable solution that I'm aware of at the moment that can provide a guaranteed accurate result... aside from native support by Designer. There is a HUGE market out there for DXF support, as shown by this thread.
I come from the manufacturing side of things, both small scale and large scale... there are plenty companies out there that begrudgingly own a subscription to Adobe Illustrator, solely for the purpose of importing a graphics file provided by a customer, sketching out the outlines, and exporting it as a DXF to be imported into CAD/CAM software or directly into a CNC machine. Designer is poised to be a perfect solution for thousands of small businesses that don't care about all the bells and whistles of AI, and just need something that can reliably generate outlines, manually correct them as needed & convert them to DXF, without breaking the bank.