I think the 84+ was supposed to end the 83+ life, but then students and parents still continued to buy them so much that TI was stuck not discontinuing them. if they did, then I'm sure they would have gotten plenty of complains from parents and teachers about even more people not being able to afford the more expensive 84+.
Btw I prefered the 83+/SE design because the ON and ENTER keys were aligned properly with 0.-, the STO> and + keys with 123, the LN and - keys with 456, etc. Now everytime I try doing 2nd+MEM I accidentally do 2nd+ENTRY, because keys are misaligned.