I think the only Mario game I'd rather play than any Bonk is SMW, and only because of its vast number of levels. But no, Bonk's Adventure did not become obsolete because of Bonk's Revenge; the games don't play the same, and Bonk's Revenge is far easier. I actually played the series in reverse order, starting with 3, then 2 and then finally 1 (Duo pack-in, I never even owned a Bonk 1 or 2 card until 2008!). I beat Bonk 3 the first night I played it back in the day...slightly disappointing but fun anyways. Bonk 2 took 3 days, but it took me 5 days of playing to finally beat Bonk's Adventure. I still find it amusing how much the Bonk 3 games go for these days.
As far as mascots go...dunno. Bonk and Sonic are pretty original characters, but Mario's kinda dull. It's always been the gameplay that made the Mario games decent, not the characters. Too bad they don't even get that right anymore. Sonic was fun for awhile...1, 2, 3, CD, and Spinball are all classics and a lot of fun, but the series took a shit bath after that. Most great platformers are destroyed by making them 3D...Mario and Sonic were no exceptions, though at least Super Mario 64 was a good game for the most part...