Sony trying to get a headstart is precisely the reason why Nintendo had to react, otherwise they would have faced a similar scenario to PS1 vs. N64 where Sony got their system on the market ~18 months earlier. Additionally, regardless of the GBA or the DS being the PSP's competitor, analysts predicted a win for Sony. Let's not revise history and pretend that it was a given that Nintendo was going to win.
The final noteworthy GBA release by Nintendo was Mother 3 in April 2006. The GBA launched in March 2001, so this aligns to five years of first party support, excluding Nintendo's localisations of third party titles that extended into 2007. That Nintendo shifted all of their support to the DS in a relatively short timespan is owed to the DS catching on in Japan during 2005 and starting to regularly pull off 100k+ weeks in Japan alone in early 2006. The market demanded that Nintendo put their entire focus on the DS. Gran Turismo 6 released a couple of weeks after the launch of the PS4; you might as well count The Legend of Zelda: The MInish Cap for the GBA in that case.
I know, but Mother 3 was the "only" notable GBA first party game since the end of 2004.. and you forget to mention that it came out Globally-in-Japan :P Mother is hardly noteworthy anyways.
Yes, you can count The Minish Cap, I don't see why not. Most preceding consoles retain a level of support when the new generation arrives. Beside GT6 (and off the top of my head), Sony released MLB and LBP3 as cross gen games in addition to the excellent third party support of PS3. Nintendo emphasized the "third pillar" nonsense to mislead GBA owners, and to make sure they had an alternative to resort back to in case the DS fails.
If Nintendo released a more technologically advanced DS a year after the PSP, I'm pretty sure it would have still been a massive success. Lifetime sales would've been closer to:
GBA lagged far behind the PSP in power, but it had its 70M~ install base advantage. I don't think Nintendo had good reasons to be paranoid to the point of screwing up the GBA owners who actually care about lifespans.
And of course NDS would sell as much as it did when it has titles like Mario Kart in its first year while GBA doesn't get anything worthwhile. They abandoned the GBA as their main handheld while pretending the DS is "completely seperate". Pokoko is correct, most Nintendo fans are indifferent to lifespans. Hell, you guys care more about Nintendo's profit than anything else, so long as their game quality doesn't go down. I mean you yourself referred to Sony's making the PSP as "bullshit" to justify the early DS release.