I created a solar power + psychic surge team to verify if solar power is unreliable or not. This team stack abilities like psychic surge, adaptability and solar power to hit really hard, i use sash excadril to set rocks up and spin, and mega alakazam (inner focus) because he is a monster under the gimmicky magic room. Despite having to run pure scarfers and having to run sash just to land some hit before dying, i found this team fun to play and even decent. Here are some replays
http://replay.pokemonshowdown.com/gen7sharedpower-705124294 (some magic room usage!)
http://replay.pokemonshowdown.com/gen7sharedpower-705128002
http://replay.pokemonshowdown.com/gen7sharedpower-705135739 (i missplayed attacking chansey outside sun and psychic terrain but in the end i was lucky)
http://replay.pokemonshowdown.com/gen7sharedpower-705140355 (more magic room, but it wasnt needed)
So, what would happen if sturdy and multiscale were both banned? The answer is: i woudnt need to have excadril on my team, making me room for sheer force nidoking. With that 30% extra power mega alakazam can OHKO chansey with psychic, sun and magic room up (it did 35% to a damaged chansey without the boosts and just magic room).
+1 252 SpA Life Orb Adaptability Alakazam-Mega Psychic vs. 248 HP / 8 SpD Chansey in Psychic Terrain: 684-806 (97.2 - 114.6%) -- 81.3% chance to OHKO
+1 252 SpA Life Orb Adaptability Alakazam-Mega Psychic vs. 248 HP / 220+ SpD Heatran in Psychic Terrain: 255-300 (66.2 - 77.9%) -- guaranteed 2HKO after Leftovers recovery
+1 252 SpA Life Orb Adaptability Alakazam-Mega Psychic vs. 200 HP / 252+ SpD Magearna in Psychic Terrain: 235-277 (66.9 - 78.9%) -- guaranteed 2HKO
+1 252 SpA Life Orb Adaptability Alakazam-Mega Psychic vs. 212 HP / 0 SpD Heatran in Psychic Terrain: 342-403 (90.9 - 107.1%) -- guaranteed OHKO after Stealth Rock
+1 252 SpA Life Orb Adaptability Alakazam-Mega Psychic vs. 0 HP / 4 SpD Magearna in Psychic Terrain: 317-374 (105.3 - 124.2%) -- guaranteed OHKO
+1 is for solar power and life orb is for sheer force. While hyper offense has counterplay even without sturdy and multiscale, with magic room i just deleted that. magic room disables focus sash, assault vest, eviolite and choice scarf, meaning that a boosted to hell mega alakazam can demolish entire teams for 4 turns without fearing being outspeed (he can speed tie mega bee and mega aerodactyl) or revenge kill by priority. And the only ways to prevent that is switch into dark types (who dies against FB or dazzling gleam), use bronzong, or stall with protect (but alakazam can use substitute for predicted protects, like guts).
If sturdy and multiscale are both banned, then teambuilding will be just stack four/five abilities and kill your opponent with that, because stall woudnt be able to keep up.
While not being the best thing that can happen, i dont consider forcing pure offensive teams to run excadril something unhealty. Most strategy games forces you to do something specific in order to survive in certain points, and that is not something bad. And it is not a limitation, a limitation would be that i coudnt break sturdy on offense teams by any mean. Also, while there are not many ways to break sash/sturdy, excadril is NOT the only option on every team: if you have technician on your team, you can use scarf cinccino. If you play contrary, shuckle can take advantage of magic bounce and bounce webs, while you can use weather to break sturdy/multiscale. Gear grind also works on kingklan teams. Forrestress can put/remove hazzards while providing sturdy/overcoat (latter is neat if you also use hail+aurora veil, and help against MB). And last, rocky helmet, ferro and garchomp can remove pressure from excadril against though claws teams with heir contact punishing capabilities.
Maybe multiscale is fine to see gone because it sinergizes really well with regenerator and PH, but i think that baning sturdy too will end up in a complete chaos.