I also have this belief. Smash 4 is just.. advertisement. The series is becoming more about relevance each release. What I dislike about Smash 4, maybe not the 3DS version but something about the Wii U version is off.
Smash is suppose to keep a balance of old school and new school.
But that's just the thing...Smash has always been like that.
In Smash 64 and Melee it didn't seem as apparent because Nintendo was still adding their major characters. Every one of the original twelve barring Samus and Ness had either a recent N64 or GB game, and EarthBound and Super Metroid were still within the last five to ten years, not to mention Samus being a major character.
In Melee, the original planned roster's newcomers consisted of Bowser, Peach, Zelda (+Sheik), and Mewtwo (all major characters of Nintendo's Big Three), Marth (the first hero of a popular then Japanese-only series), and two token retros (Ice Climbers and Game and Watch). When the clones were added we got additional versions of Mario and Link, we got the main villain of Nintendo's third biggest franchise (Ganondorf), we got Fox's second in command (Falco), and we got two purely advertisement characters (Roy for The Binding Blade and Pichu for Gold and Silver).
Brawl's entire newcomer roster came from recent games (funny enough, even the two token retros) as Swamp pointed out.
It isn't a new issue. What it is is that it seems more apparent now, and that's because we're at the point where almost all of the bigger, important characters with long legacies are already in the game. At this point we're scrapping the bottom of the metaphorical barrel. Even then, we got quite a few characters in Smash 4 that were more than just advertising.
Rosalina and Bowser Jr. had become Mario mainstays. Palutena may have only gotten considered because of Uprising, but she'd been around since the eighties. Little Mac is from a series that was incredibly popular in the arcade and NES days, and is still popular today. Duck Hunt is the token retro, and much more well known than our last few token retros, I'd say. Villager is the main character of a series that's been big since the GameCube, and N64DD for Japan. Miis became cultural icons when the Wii launched. Mega Man, Pac-Man, Ryu, and Cloud had storied histories and are some of the biggest names in gaming. Wii Fit Trainer was a surprise pick, yes, but she wasn't made to advertise (hell, Smash's demographic doesn't even overlap with Wii Fit's in the slightest). She was from a best selling series on the Wii.
Then we look at the ones who are left: Greninja, Robin, Lucina, Corrin, Shulk, Dark Pit, and Bayonetta.
Robin, Lucina, and Corrin come from a series that doesn't keep a static cast, so putting in the newest hero just makes sense, like it did with Roy and Ike in Melee and Brawl. Lucina was also a last minute bonus clone. Greninja is the same case as the Fire Emblem crew. It makes sense that they'd want a Pokémon from the newest generation instead of, say, going back to Hoenn and picking someone like Shiftry.
Dark Pit was a bonus clone from a recent game, like Lucina, and he initially wasn't even his own character. Hell, he's the only newcomer who didn't get a trailer, so if he was an advertisement, they for sure did a poor job. Shulk was the main hero of a successful new IP that had just recently gotten localized due to fan demand, just prior to when Sakurai decided on the roster. Bayonetta is a different case entirely since she wasn't Sakurai's choice, but the winner of a fan poll.
Smash has always acted as an "advertisement." Now, however, all of the truly evergreen characters are already in, so the way rosters are chosen tends to sway toward newer characters. Smash was never about "balancing new and old."