When will Acrobat Reader stop supporting XFA forms?

I don't work for Adobe and don't speak on their behalf, but considering some very large clients of theirs use this type of form extensively (IRS, banks, governments around the world, etc.), I doubt that's going to happen any time soon. Why kill a goose that's laying golden eggs? What makes you think this is a possibility, anyway?

I don't know exactky why you say "Upgrading to AEM forms is not an option". But AEM Designer, the successor to LiveCycle Designer, continues to make XFA forms. Adobe rarely share their future plans, and they have not accounced any plans to phase out XFA.

That said, I woudl not advise anyone starting with forms to make XFA forms, even if they have Designer. My reasons

1. The spread of non-Adobe readers means end users face more challenges to use them.

2. The refusal of Adobe to implement XFA on mobile devices means a huge proportion are just locked out anyway.

3. Adobe's plans can be seen to be focussed on the AEM Forms server product, which converts XFA forms on the fly into HTML5 forms.

4. There are so many obstacles to PDF forms of every kind I advise HTML forms for all online use.