this is an observation that has not been lost on me.
So what would the problems be if multiclassing was enforced and the system reshaped to support it by making every class, base and PrC, 6 levels long?
the only issue I have with this is .... what's the point? all you've really done is identify the basic economics of D&D character building. it's all about
opportunity cost, and instant gratification vs. delayed gratification.
opportunity cost is simply the relationship between
scarcity and
choice; where scarcity=class levels, and choice=what you do with those class levels.
Where choice is highly valued (which is often the case in 3.5), then simple economic theory dictates that you just let the player decide how he/she wants to spend those class levels -- your observation simply illustrates the scope of importance tied to that choice
BAB is overrated IMHO (both in general and in regard to this particular issue)
meh ... a bit of a strawman, IMO:
Many builds don't need BAB
casters that don't use touch spells. niche.
Many others have ways of making BAB irrelevant (like Divine Power)
even more niche: clerics of level 7+ ; it's big brother is level 9+ ; it's arcane equivalent is level 11+. and all of them are short duration (without silly persist
shenanigansacrobatics), which makes them even less useful. hope your spell doesn't get disrupted/dispelled.
Others plan on only taking full BAB classes, so staying 1 level more in the base class is not beneficial at all to them (BAB wise)
rogue/monk/scout/ninja/any number of skillmonkey classes/PrCs?
Interesting point about saves though, was unaware of that
Even if IMHO in most cases +1 to all saves is not worth delaying PrC features for a level, in builds that need a 'filler' PRC at the end, it might be worth taking your 6th base class level before entering that.
this is dependent on what PrC features you'll be gaining. additionally, many times you'll also be gaining a class feature from your base class (or at least an upgrade to an existing one). making this quite situational.
I'm not necessarily saying that you're "wrong" ..... I'm just pointing out the holes.