Actually, I see the exact opposite is the case. The harder a base is to take the longer it takes.
In theory, yes. Practically there is a limit to that. make it too easy, and no one can and will defend anymore. Make it too hard and you will quite often not see a prolonged attack, but an aborted one in case any serious opposition shows up. There is a limit to the stamina and stubborness of the attackers. One may lament about that, but that's how people are
A prime example for that was the EW arena. After we got the new big town, not only the general population, but the number of kills (=combat!) in particular went down drastically. (charts will follow). It simply was too hard now, so people stopped trying and went elsewhere. When we got the flags in town, not only the milkrunners came back but also the number of kills skyrocketed again (relatively spoken).
The effect on LW wasn't as drastically, but very noticeable. Another point was than with a greatly reduced capture rate, there was almost no more map rotation and no more won wars. And if we like it or not, this is a very important part in gameplay and long time player motivation, especially for newer players.
Somewhere there is a (blurry) line between, "Come on guys, one more try" and "Screw that, we don't have a chance". I might add that we still don't have as many base captures as we used to have over the years, mostly due to the changed arena format.
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I can imagine a gameplay setup in which the individual base capture is much harder to accomplish, but you have to make a lot of other adjustments on several levels (from map design to war victory conditions) to make that work without too much overall frustration