MGSV tried to take a middle ground between the two "open world" game design philosophies, and didn't do it very well. It had sandbox-style maps, but little sandbox-style content, but relied on the older style of mission-free roam structure and content gating. Mother Base itself wasn't the problem, but rather that little was done with it except for a couple missions, and serving as the venue for progression mechanics. Honestly, I think the game could have pulled it off -- if the revenge system/enemy preparedness mechanic had been a lot more robust, aggressive, and dynamic. Those big-ass maps would have felt a lot more claustrophobic and dynamic if Snake (and the player) had to contend with infantry/armored patrols, CAP's, lookouts in unexpected positions whose sole purpose wasn't to engage Snake but rather report back intel on his whereabouts, CP's and outposts which actually upgraded in hardware deployment and fortification (or were abandoned, to deprive enemy forces of resources) based on Snake's activities, and even locals who could be allied with or turn against Snake depending on his choices.
For instance, I remember Afghanistan CP's 6, 9, and 11 to be my favorite whipping boys when I played MGSV. Centrally-located, placed along natural chokepoints, easily approached and vulnerable, and always there brimming with potential recruits and supplies. No reason to not hit them, ever, if you were traveling between major locations. So, what happens, and how does it impact your gameplay, if you've been a little aggressive, and ride through to unexpectedly find CP 11 and 6 abandoned except for one lookout on the ridges, and a tank company with heavy soldier escort and extensive fortifications at CP9, who could in turn rapidly respond to reports of enemy activity at Wakh Sind or Da Ghwandar?