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Yes, Nevrus. 
That is precisely what I am afraid of.

If this is indeed the case, then that does indeed make Ambush a buy-in feat.

Currently, there are very few ways to gain advantage.   Attacking while hidden is the only way to gain it without purchasing a feat.   The rest of them require spending a feat to gain advantage in an extremely limited situation, or at high levels (two weapon strike).

Your interpretation of maintaining "Hidden" status, and thereby maintaining advantage, seems reasonable -- but it points us in a direction where for the vast majority of situations, your non-Ambush rogue simply cannot move.

If your interpretation is correct, then for the vast majority of situations, taking the effort to hide isn't going to really be effective to grant advantage *without* taking the Ambush feat.

Take, as an example, the halfling ability to hide using man-sized creatures.   He could take his action to hide one turn in the midst of combat, but if he moves at all, he is spotted, and gains no advantage from it.   The only way this would be useful or functional under your interpretation is if he never moved at all, and poked his dagger *Only* into creatures that moved within his range.

Without taking Ambush as a buy-in feat, under your interpretation, that halfling hiding ability is, for the vast majority of situations, practically useless.

This interpretation limits Hiding to really only being useful in situations of heavy shadows and obscuring fog, etc., as one needs paths-of-movement which also maintain Hidden status so as not to lose advantage on the attacks.

Thus, all of these highly-stringent interpretations negate the utility of Sneak Attack in general, as it begs serious feat buy-ins to be able to activate it. 

False. There are more ways than that to gain advantage. You have to remember the rogue is a very martial character. As such, combat techniques and maneuvers can help, as well as equipment.

1. Prone- Anyone can attempt to knock a creature down as a strength contest. A fighter with the Trip maneuver (Or a Rake rogue with it) can do so as part of an attack. Ball bearings can make creatures trip during their movement, falling prone.

2. Restrained- Hunting traps, grapples, and various wizard spells can restrain a target, making it grant advantage to attackers.

3. Paralyzed- Hold Person/Monster can make the target fall prone and grant advantage.

4. Blinded- Various wizard and cleric spells can blind a target and make it grant advantage.

5. Drunk- Yes, you get advantage on drunk targets.

6. Feint Skill Trick- One on one, this allows the rogue to get advantage.

There are plenty of mechanics in place that will allow the rogue to gain advantage. It takes party coordination, though, because rogues are inherently dirty fighters. One-on-One, there's not a lot they can do to outmaneuver an opponent, but it's possible through laying traps.

Playing a rogue is a strategic game about getting the upper hand on your enemies. There aren't a lot of ways to gain advantage and attack with it as a single turn- it's a coordinated effort for a reason. This is a design choice because advantage is so powerful, especially for rogues.