"Unlike other special rules, Frenzy can be lost as the game goes on. Models retain their Frenzy for the entire game unless beaten in combat..."
Let's break this down:
Unlike other special rules, - Now we know we're discussing something out of the ordinary for special rules.
Frenzy can be lost as the game goes on. - Frenzy can be lost during the course of the game where under normal circumstances a special rule would still be in effect.
Ok pause here a moment. That sentence says (putting it together): Unlike every other special rule, you can lose Frenzy during the course of a normal game where under normal circumstances a special rule would stil be in effect.
Normally a special rule, if it's on your profile, stays with you the whole game unless lost to a spell or item. A special rule granted by a spell or item lasts as long as the item/spell is in/active on you/your unit.
So common sense check: Does the fact that you can lose Frenzy affect the normal rules for having a special rule? No, except for what is specifically stated, that you can lose it by being beaten in combat.
Continuing the break down:
Models retain their Frenzy - Note the use of the word "retain." To retain something is to keep possesion of. You have to already have something to retain it. We'll come back to that.
for the entire game unless beaten in combat - Now we know how long we retain it - the entire game - unless we lose combat.
Let's put that sentence together now: You retain posession of this special rule for the entire game unless you're beaten in combat.
So now let's put the whole thing together: Frenzy, unlike other special rules can be lost where under normal circumstances a special rule would still be in effect. You retain this special rule until you are beaten in combat.
You have to have something to retain it!
How do you have a special rule:
1) It's on your profile.
2) It's provided by a spell.
3) It's provided by a magic item.
For #2, you only have the special rule for the duration of the spell. For #3, you only have the special rule as long as the item is within the relevant area of effect (on the character, in the unit, within 6", whatever it may be).
For #1 though, you always have it. Hence the important note that you can lose Frenzy unlike other special rules.
The point is that Frenzy is unusual in that you can "erase it" (lose it) from a model or unit when any other special rule would still persist. How do you lose it, you are beaten in combat. If that condition does not occur then you continue to retain it. If you don't have the rule anymore, then the discussion is moot as there is nothing to retain posession of!
Cherry picking a portion of a rule out of context is generally not a good idea in a large, complex system.