Swap the phrase "a replacement" with "more options". You now get a choice of which red dragon and kobolds suite the encounter/adventure/setting better. Is this really a bad thing?
Why would you want to use the absolutely terrible, fundamentally and mechanically flawed solos of the original MM? This isn't a choice between an essentials done Red Dragon and one from the original MM. This is a choice between something with 2 years of updates to monsters and one that is
fundamentally broken.
Noting that most heroic monsters from the original MM are in fact still okay. There are some stupid things (Bloodfire Harpies and Needlefang Drakes) but in general they are functional. New Kobolds are new options, because many MM heroic tier monsters are still perfectly viable (Dragonshield Kobolds for example).
Now when we get into the paragon and
especially epic monsters - many of whom are so useless not even upgrading their damage helps - we have a problem. Most MM elites and solos are worthless after heroic tier. Orcus is arguably the biggest joke in all of 4E for an epic level end of game threat. Most regular MM high paragon and epic monsters are nowhere near competitive with an epic party (damage and power wise). Compare high level monsters from MM3 like Forsaken or Tulgar with just about anything high level (level 21+) in the original MM. Heck compare similar epic tier monsters in MM2 to MM1. MM2 creatures have better power design, so frequently all you have to do is change damage maths and bam, you're done.
I mean, what Wizards are saying about the PHB and DMG is 100% true. What they are saying about the MM just rings absolutely hollow to me. It's a fundamentally broken book, with many creatures being useless (minions, many solos, some elites) and others being so far behind the curve they aren't worthwhile using. Of course it's worth noting I feel this is
only for most creatures in paragon and beyond. That's where the gaping flaws show up and those are the creatures that perform the worst.
Since the MM3 has come out I've used monsters almost exclusively from it and the MM2 (with adjusted maths). I rarely use creatures from the original MM after heroic tier at all and I never touch the solos in the book.