Well, yes, the clear logical choice is to stay happy w your current i8.
My choice for a slightly used 2019 coupe was based on my need for a backseat, the slightly better handling / weight, and 4+ years unlimited miles warranty. I compared a number of i8s back to back to back. I realize it is hard not to be biased, but I convinced myself driving the same route several times that the 2019 was a modest but noticeable improvement over earlier models; crisper handling, better dampening response, and trivially (.2 sec quicker 0 to 100kph) better timed acceleration. I like the interior and exterior cosmetic changes.
But key for most of us is value. A 2019 is more expensive initially, but also retains more resale. So it is the difference in relative depreciation and maintenance/repair costs including time and aggravation. Plus the loss of appreciation of capital that you might have had invested in the stock market. There are many days when I think I should have been content with riding my bicycle to work...