Does anybody know a way to suck all the content out...
As Groups.io got a mention earlier (I would have mentioned it myself but didn't as I didn't want to "advertise") I'll mention it again.
Groups.io will download all your Yahoo material* and load it on to a new group once you have set that new group up. Note, however, that there is now a charge for this (and has been for perhaps 12 months) so 1 year of "Premium" Service has to be paid for up - front at (IIRC) $110.
If your total storage requirement exceeds 1 GB (which does not include the message archive) then you will need to pay for an enhanced service of one type or another in perpetuity, either the "Premium" rate as above (10 GB) or the "Enterprise" rate of $2200 per annum which will get you 1 TB of storage.
I've never seen it written down but I suspect that if you need > 10 GB at the outset then you will need to pay for an Enterprise Group from the very beginning.
* At the time of writing accessing Photos on Yahoo Groups is reported to be rather hit and miss.
And apologies if this looks like an advertisement, but someone did ask...