If you bought an iPad in 2012, it should have received software updates until 2019, and all iPads since 2015 are still supported. In general I expect the first 2-3 generations of a new product line to have shorter support cycle before they have matured a bit, so it doesn't surprise me that your particular iPad model only got 7 years, but if you buy an iPad today I would be surprised if it got less than 10+ years of support.
All that said, I think hardware companies should be forced to open up their hardware for booting alternative OSes once they are classified as EOL.