Honestly, even if this happens, taking down structures makes up for it. It doesn't in my experience, however.

It doesn't.

If you took down both outer cannons and a fort, you'd be up 1600 xp (400 per cannon, 800 for fort).

Every minion wave at level 1 is worth ~450 xp, a hero kill at level 1 is worth 300 xp.

If all 5 of your team goes bottom, and the other team puts 3 bottom to defend while leaving two in other lanes, you're going to be losing 900 xp per minion wave* (450 in each lane). By time two minion waves have gone by, the other team will have an ~1800 xp advantage on you. Even if you manage to get the cannons and fort down, how many minion waves is that gonna take? Let's say you do it in 3 minion waves AND manage to get 3 kills at level 1, 2, and 3. (Assuming the other team is the same level, you get more xp per kill than the baseline if the other team is higher level than you). So you'd have lost 2700 xp to not being in 2 lanes, and would have gained 1600 xp from killing structures, and an additional 300+350+400 for killing heroes, or 1050. Compared to the other team's 2700 experience, you'd have gained 2750. And this is a rosy best case scenario.

Most teams are PERFECTLY capable of defending a 3v5 push with the right heroes, and the chances of you actually killing two cannons and a fort in 3 minion waves is probably impossible. Not to mention, leaving a lane undefended will pretty much ensure that the solo heroes are going to kill your outer cannons as well, for 1600 more experience for them. (4 cannons, 2 in each of the other lanes).

In a realistic scenario, if you manage to kill a fort in 5 minion waves, you'd have lost over 4500 experience by not being in lane, for a gain of 1600. You'd need ~10 kills in that span to bridge that gap, and that's not even including the other team killing your cannons. That's also assuming no one on your team dies at all in that 5v3 push.

In short, this is a terrible strategy.