I was wondering where this product was, and/or why some of the podcasts I listen to don't use something like this.
This works on a browser, though? What if the guest forgets that they still need to upload the tail end of their data and closes their browser?
(I'm not a podcaster so this is just curiosity)
These are called "multi-enders", and lots of podcasts do them. There are many[1] tools for this which are focused specifically on podcasters.
Additionally, it's straightforward to do it manually as well. (Tip: Have participants on all ends do a "1-2-3 <CLAP>" to make it easy to align the audio during production. Do this at the beginning and end to account for digital clock drift.)
[1] Cast: https://tryca.st/ [2] Cleanfeed: https://cleanfeed.net/ [3] Ringr: https://www.ringr.com/ [4] Zencastr: https://zencastr.com/
We have an uploader system. So if they quit too soon, you can always send them a link so they can finish the uploading.
Generally browsers will let you store a few GB of data (depending on the user free space) if your really wanted to but it's not recommended since they can delete it any time.