I'm not seeing it being a big issue, at least by rough math.
Typical Damage Loadout of a 5th-Level PC
Based on DMG Page 42
E 3,1 + D 5, 1 + U2
Assume a D = High Limited, E = Medium Limited, U = Medium Normal damage expression. This is *really* rough, but seems in line with powers as far as I can tell.
55% hit rate
Daily 5 = 3d10 + 4 = 7-34, mean 20.5 = 11.3 hp
Daily 1 = 3d8 + 3 = 6-27, mean 16.5 = 9.1 hp
Encounter 3 = 2d10 + 3 = 5-23, mean 14 = 7.7 hp
Encounter 1 = 2d10 + 3 = 5-23, mean 14 = 7.7 hp
Utility 2 = 1d10 + 3 = 4-14, mean 8.5 = 4.7 hp
Subtotal before at-wills = 40.5 hp
5-character party Subtotal average damage yield = 202.5
Up against Young Red Dragon (Solo Soldier 7, 332 hp), they have it at 129, well past bloodied before they’re out of limited powers.
Now they have to use at-wills. At-will = Medium Normal, 1d10 + 3, mean 8.5, 4.7 output per attack
Per-round damage output for party using at-wills only = 23.5 hp. It will take them 6 more rounds to wear it down.
Yes, that could get dull.
Now, do 2 things.
Increase the hit rate to 65% (equivalent to a +2). This reflects cooperation for flanks, going after vulnerabilities, etc.
Assume that you get one good solid Medium Limited result from an environmental hazard or other encounter-specific resource per PC.
New Subtotal for limited resources
Daily 5 = 20.5 = 13.3
Daily 1 = 16.5 = 10.7
Encounter 3 = 14 = 9.1
Encounter 1 = 14 = 9.1
Utility 2 = 8.5 = 5.5
Hazard = 9.1
Per Character Output = 56.8 * 5 Characters = 284
This takes the dragon to only 48 hp remaining – just over 2 rounds of at-wills.
We’ll add 1 round of “overhead” for non-attack actions, so it takes 9 rounds to take it down. What can the dragon dish out in that period? About 12 hp per attack, so a total of 108 assuming 1 attack per round. Actually, being a solo, it’s going to be more like 2 attacks per round, so call it 216. Advantage: PCs.
A well-built solo encounter should be pretty tough, but not too bad of a slog for PCs who work well as a team and use encounter features to their advantage.
Go ahead and pick apart my math, though - I'm only so-so at this sort of thing.