The game needs a grinding process to make people want to high tier, and gears should 'solve' lower tier problems.
This is what 'traditional' MMORPG doing.

No one posting T3 3 and 6 men instances on world chat, and few want to do it.

An ideal way could be:
Each tier has 25% damage/healing/morale increment, mobs hits 20% harder and 20% more HP, some mechanics will become more dangerous. (For example, orc archers in TG throw random-targeted heart-seeker, no CC or interrupt = at least 85%HP damage on suitable geared dps, low-geared dps will be one-shoted)
High tier instance won't take longer time, but mechanics requires players to become more professional(careful).
You can kill mobs fast, but if you do not use potions/avoid damages/Daze or CC down dangerous mobs, you will wipe and fail some challenges. (For example, time challenge quest and embers)
And in my opinion, 3 men and 6 men instance should be quick, can be finished in 30 mins/1 hour with a decent group.
Comparison with raids, small instances should for casual players.

Without these, high tier merely kill people's time, it is normal that no one interested in these.

My suggestions:
More, higher gear level
Higher essences from embers, but requires tier-x deeds.
More 'Challenge' quests in low tiers reward embers, for example, daily challenge quests, finish without retreat/boss reset, in a time limit, each reward 50 embers. -> Make high geared players also want to do low tiers.
Fix the DOT table bug in Cavern (20000+ on t1 is a broken) .