The thing is, modders are Bethesda's most die-hard fans, and Bethesda games have some of the largest modding communities in the world. Many of these modding projects have taken an extraordinary amount of effort from an extraordinary amount of people. For them to not communicate and work with the community at all is a big slap in the face because modders have given people a reason to continue purchasing their games and keeping their business relevant.

Is Bethesda obligated to work with modders to try and help with backwards compatibility? Not legally, no. But their lack of effort is rightfully leaving a sour taste in many people's mouths.

To your last point, I'm sure you didn't mean it intentionally but that is a straw man argument; nobody is claiming that every single mod needs to be compatible with Bethesda's updates. However, if Bethesda at the very least communicated and worked with the community even a little bit, they could make the task of updating mods substantially easier. Instead, they chose not to, and now countless of hours of work will be wasted because so many people will not find it worthwhile to update their mods to be compatible with the new version. It's extremely unfortunate.