In the game, it comes down to what one can do for the
time one has spent on the game. Or, crowns for the upgrades
One can spend an eternity in IA and max out everything including university researches, and then take out rushed Atomic Age bases. Or, one can instead spend the
same amount of time in focusing on offensive upgrades, maybe some minimal defensive upgrades, and maybe no economic upgrades, get to Atomic Age, and then take down said or even better Atomic Age bases.
It's a choice each player has.
Levels can be highly misleading. Correlating levels to ages to make decisions about the usefulness of a player in wars can also be highly misleading.
If the levels come from road upgrades and other economic upgrades, and if the troops are weak and are behind by more than one age, the said player might not be a great addition to a war roster. (One age allowance is only because one cannot know if the player just made it to the current age a day or a week ago, and might be in the process of upgrading the troops).
On the other hand, if most of the XPs for levels of a player come from offensive and defensive upgrades, and if the player has ignored economic upgrades (including roads), the said player might be a valuable addition to the war roster. Even if his/her level is low for an age compared to the general average for that age.
Sure, someone who has moved ahead through the ages by focusing on offensive upgrades and with minimal defensive upgrades might get 5 starred every war by an opponent of the same age or sometimes, an age lower. But,
it would be no different had the player spent the same amount of time in the lower age, trying to max out everything. The opponent player who 5 starred him/her would still 5 star him/her in this scenario.
For example, if a player has played for about a year, and has reached Global Age with all of the upgrades to offensive capabilities, but, with minimal defensive upgrades, the player would get 5 starred by other Global Age players (or, sometimes, Industrial Age players). Granted.
But, if the same player had spent the same amount of time trying to max out every upgrade and is at EA or early IA by the end of one year, the base would still very likely get 5 starred by the same Global Age player who 5 starred him/her previously. This doesn't make the player less valuable.
The difference between these two cases would be that the player who focused on offensive upgrades and minimal defensive upgrades will have better troops for attack (bazookas, howitzers), better tactics (access to decoys) and more tactic slots, better mercenaries, and would perhaps be able to add a missile silo to the base, and might be able to throw off some opponents off guard if they weren't careful. And, the player would be able to bring in a lot more punch to the attacks by being on the war roster.
These are choices players have, and one approach isn't definitively better than the other.
The only bad approach would be mindlessly and badly rushing through the ages, by gaining the minimal level requirements to advance an age through economic upgrades, and significantly lagging behind on upgrades to offensive capabilities.
In summary,
return on time invested, viewed in conjunction with choice of upgrades a player has made are better indicators of how useful a base is in wars, instead of going by absolutes on age or levels.
I can understand that the extra firepower from players who have focused exclusively on offensive upgrades might not be too valuable to an alliance if there are already enough heavy weights (who have spent a lot of time, or a lot of crowns, or a combination of both) to cover for all bases in a war by virtue of having two attacks each. But, the vast majority of the alliances who don't have a lot of heavy weight players would find those who focus on offensive capabilities and move through the ages to be valuable additions.
Hope this helps.
Cheers and happy gaming!