Crippling Keep is neither good nor simplifying.
If you were using Keep as a kind of reminders/tasks list, you should switch to Tasks for that, since it's much better designed for that use case.
From the product POV, anyone who has ever used knowledge base will tell you that you should keep everything related to your data/ info in a single app. Spreading them out by any categorization is just inviting recall issue later on.
Tasks in GMail (and IIRC also Calendar) existed long before Google Keep. You are talking about the standalone application of Googles task-system which came with version 3(?). If anything, Google Keep is the problem here. Why was it duplicating features which were already existing in the Google-verse? Instead of integrating with the existing systems and improving them.
There is nothing wrong with spreading out your interfaces and workflows, as long as they all work on the same data. No interface is perfect, so having specialized interfaces for specific jobs is beneficial. But Google is not doing this with Keep, instead maintains another parallel system again.
I have a few recurring Keep reminders, which is a document larger than a couple of sentences that contains line breaks for formatting.
Keep reminders intuitively made sense to me. They're sticky notes that you can label/index/search. Reminders allow you to be reminded of a specific note, which I find tremendously useful. Also location based reminders are great, e.g. grocery list that pops up when you're at the store.
"Tasks" suffered from the obvious- they were integrated in Gmail through labs and then calendar widget, but then keep reminders and assistant reminders appeared on the calendar. It was confusing. Keep makes it relatively simple, though you're right, it can be deprecated soon. Still not sure I'm ready to use "Tasks" yet.
Tasks are things that one should do.
A reminder reminds the user of something at a specific time or perhaps at a heuristically chosen convenient time in a range.
So a satellite flying directly overhead or someone’s birthday or simply wanting to be reminded of a fact on a day is not a task. There’s nothing to do, and checking it off makes no sense. (Although wishing someone happy birthday could be a task.). Simply showing the user the reminder accomplishes the goal. On the other hand, most calendar programs struggle with events with no duration, and overlaying reminders onto a schedule makes the schedule harder to read.
I get that sometimes decisions have to be made about whether to keep or abandon design and UI and nomenclature decisions that are now regretted for whatever reason, but bear in mind that you are creating immense frustration for current users when you do so.
As the designer or manager or developer of an application, you are immersed in it and every change makes perfect sense to you. But your users may use it infrequently and all your changes are just seen as pointless, frustrating churn.
A birthday is a repeating calendar event. You're free to create a calendar event with 0:00 duration in Google Calendar, it works fine. You can also create all-day events that show up at the top and don't make the schedule harder to read.
Calendar is not going away, calendar events are not being touched, this has nothing to do with alerts for calendar events, and this does not require you to use a new/separate app.
Google has simply had different things called both "reminders" and "tasks" across apps, for legacy reasons. For a long time there's been a standalone "reminder" object in Calendar (different from events), and then a "task" object got added as well (that shares data with the Tasks app). There's no good UX reason for these to be separate types of objects, so now "reminders" and "tasks" are being consolidated into just "tasks". But they're staying in Calendar. (And can also be edited in the Tasks app which has been around for years already.)
This is good. It's simplifying.
(As to why they're now splitting off reminders created in Google Keep rather than integrating them too, I don't know for sure. But I've never understood the use case of reminders in Keep at all, so I wouldn't be surprised if they deprecate that feature entirely. Keep reminders came before Tasks existed at all, but now you should just use Tasks.)