While I do agree that laners should leash, I do not believe it is as big a deal as you are making it to be. This is due to a few main points.
[1] Sometimes your top-laner cannot afford to leash.
While this is less of an issue since double-jungling is no longer standard (and or a viable stategy at all), many laners needed to be in lane before the minions even met to shove the wave under the enemy tower and deny their opponent experience (and to get a free base).
Sometimes, the top laner will need to shove the minion wave under the enemy tower immediately unless they want to be crushed in lane (either due to levels or playstyles). One example of this would be Singed: he needs to either start proxying the first wave or shove it under tower immediately (even missing CS) and proxy in many matchups or he will be crushed.
[2] You should be able to clear without a leash.
While you won't be quite as healthy, you should be able to perform a leashless clear of at least three camps on any jungler you play. If you can't clear at least three camps without a leash, then it is quite possible that you either need to adjust your setup (runes, masteries, starting items) or you shouldn't be jungling with that champion in the first place.
[3] Junglers have to be flexible.
In many cases, junglers may have to start from an unfavourable position due to circumstances of the game.
- A level 1 fight may occur and your allies may have to rush to lane or base
- You may have to start away from your preferred side of the jungle for strategic reasons (i.e. start top lane to gank bot early, starting on the opposite side of the jungle to avoid an invade)
- The enemy jungler or support may invade you on your first couple camps (either stealing it, delaying you or chunking you down)
Stuff happens in games and sometimes you just have to deal with it.
Also, slightly unrelated but a useful side-note. Amumu is perfectly capable of beginning on red-side and doesn't run into any issues when clearing. Amumu isn't really mana-reliant for clearing with Hunter's Talisman and Hunter's Potion if you aren't spamming your Q on camps.
Attack Speed marks speed up your clear and make strengthen Krug buff while W provides great damage for low mana consumption.
You can easily clear Krugs, Red Buff, Raptors, Wolves and Blue Buff with W and Talisman without issue.