Nokia thought that phones would be like cars. Most car companies have dozens of models with different characteristics and prices, and that's fine. Even BMW has 20(!) top-level model categories. Nokia was selling devices all the way from high-end camera/phones for $$$ right down to $10 (unsubsidised) with two week battery life for emerging markets. I think you can argue that 50 was too many, but 10 or 20 was reasonable and still is - plenty of successful Android OEMs have ranges like that.
The real shift in phone design and in the range was that the screen took over the whole front of the device. That meant there was much less scope for different shapes and form-factors, and since you were no longer using most of the front for casing and keys there was less to design anyway.
Meanwhile all the actual phones ran either Series 40, the classic feature phone OS that won them half the entire market as 'easy to use', or series 60, the smartphone OS, that was an actual platform but problematic (and fragmented) in lots of ways.
Personally, I think Nokia didn't really think anything. The management was just incompetent and they didn't have any real strategy. They had 100 teams of engineers each doing their own phone model, and were just too lazy to fire unnecessary teams and focus. Somewhere deep down they knew, that it is stupid way to do things. However the money kept flowing in, so who cares.
It was also slow and bloated... and china happened. Nokia guys would start making a mockup and a powerpoint for a new product, meetings, comittees, etc., and in that same time, the chinese would start selling the actual phone.
This was it for me. I gave up on Nokia when everybody around was using their Android messaging apps while I had no apps for my Nokia, or they were a lesser version (like whatsapp).
Then, if I recall correctly, they went with Windows, so every move was forcing their users into a lonely corner.
Samsung is successfully using this car company strategy and they have a phone in almost every price band. Just look at sheer number different SKUs they have: https://www.gsmarena.com/samsung-phones-9.php
The real shift in phone design and in the range was that the screen took over the whole front of the device. That meant there was much less scope for different shapes and form-factors, and since you were no longer using most of the front for casing and keys there was less to design anyway.
Meanwhile all the actual phones ran either Series 40, the classic feature phone OS that won them half the entire market as 'easy to use', or series 60, the smartphone OS, that was an actual platform but problematic (and fragmented) in lots of ways.