If you don't want to fast forward the lower levels, but still want people to group for the fun and learning experience of it, you need to find a solution which makes grouping at the lower levels more frequent. For this we need to look at the reasons *why* people don't group at the lower levels.
The problem has two parts: grouping at lower levels nowadays (compared to the time when World of Warcraft came out) is more difficult because there are less people of the lower levels around. And second it isn't profitable enough to group to be worth trying to overcome this higher barrier into grouping. The solution thus has to be to make it easier to find a group, and to make grouping more worth it than soloing, even counting the time it needs to set up the group.
One interesting solution I proposed before was to create cross-server dungeons. If cross-server battlegrounds can work, then why not cross-server dungeons? You'd need to put up the same restrictions on trading, but otherwise it should be possible. You just need a queuing and automatic grouping system like battlegrounds have. A bit more elaborate to make sure each group has a tank and a healer, but otherwise similar.
But that alone wouldn't suffice. You'd still need to increase the rewards for grouping to make up for waiting in a queue and potentially being grouped with nitwits. I'd keep the gear rewards as they are, the loot dropping in low-level dungeons is already good enough. One could think of adding something like a badge system, giving every player some sort of points or tokens to buy gear with in case nothing useful for him dropped. But otherwise the dungeon loot is okay as it is. What needs to be improved is the xp you get in a group. Right now the xp bonus for grouping is small. The xp for a mob is divided by the number of group members, and then some small bonus is added, around 20% if I remember correctly. Unless you have the world's most efficient group gathered together in zero time, your xp per hour in a group are notably less than your xp per hour when soloing. That shouldn't be the case. Joining a cross-server dungeon queue and doing a dungeon with a bunch of strangers should on average bring more xp per hour than soloing. Nobody wants to eliminate soloing from the game, it should always remain a viable option for people who don't have the time required to group. But soloing shouldn't be the fastest way to level up, because that teaches people to *not* group, which is counterproductive.
MMORPG players in general react strongly to rewards. Making grouping more easy and at the same time more rewarding can save the low-level game from irrelevance. Expansions should expand the game, not shorten it or make the old content useless. Cross-server dungeons with higher rewards could well keep the old content alive for new players and veterans with alts alike.