Yup on the gravel vac... where "live" is concerned, I tend to agree.
Larger grain sizes don't require as much (if any) stirring or turning, so larger infauna are not that important. However, when you start getting into sands, especially oolithic substrates, things get a little different. Where gravel may have almost no anaerobic activity at 2-2½", oolithic sand already has a substantial anerobic zone. The processes that occur can means pH can vary from water column pH at the surface of the bed down to -.4 at a depth of 2½" (tested). At this point, some substrates are no longer inert and serve to buffer (melt into solution) a bit. This, combined with biofilm, can serv to cement grains together and form a crust. If this happens, it is possible that the upper and lower zones can become sequestered. This can lead to a DSB "crash". For this reason, in terms of defining the term "live", most advocates of "live sand" do not consider the bagged bacteria stuff as even remotely live. The substrate requires a SLOW and steady movement through it which can only most easily be accomplished by larger benthic organisms or fish.
The turnover, as OinKY has said, needs to be a slow, steady, natural process. The aquarist meddling in affairs introduces oxygen into the anoxic layers, killing off much of what as there. And worse, aerobic organisms are pushed down into the lower layers where they'll also die off. This is one of the primary reasons why FW only aquarists sometimes have trouble believing how a "more difficuly" marine system can be readily kept WITHOUT a filter of any shelf or homemade kind. An aquarist I know from another board calls it "voodoo filtration". Simple fact of the matter is that, if you have the diversity, if you do it properly and stock reasonably, you don't need a "filter" any more than a small lake, pond, or even coral reef, does. Life IS a filter as long as you're not constantly trying to sterilize or partially sterilize your tank. This would also include ANY treatment used to discourage any form of life (i.e. algacides).
One of my biggest peeves in the hobby is "algae problem". Algae in tanks is NOT a problem, it is a symptom of something else that may (or may not) be a problem for the pivotal occupants (usually fish). Algae is life seeking balance, life creating its own filter because no other means of filtration is allowed to occur that would lower the available nutrients feeding the algae. With deep substrates, you will almost always find less algae as nutrients that would fuel the algae are used up by the bed instead, never allowing algae to gain a reasonable foothold.
MOST experienced marine aquarists do not go with SSB (shallow sand bed) systems because of this. The SSB is not near as efficient. If the stocking load is not incredibly light, the SSB will not be able to keep up and algae and cyano maintenance can become a neverending chore. This is why most marine tanks are either fairly deep (3-6") or BB (bare bottomed). And I can confirm that any time I've tried a sand bed of 2-3" or less, cyano has required routine maintenance and (which I abhor doing in SW) regular water changes. Maybe not at first, but eventually.
On the FW end, my 135 has recently sat for about a year with 0 maintenance other than top off and twice daily feeding, no filter cleaning, no water changes, nothing. Did it look a little rough around the edges? Well, that would be an understatement, I'm still trying to get some mulm away from the Val roots. But it's not overgrown with algae, and the (pretty ancient for their species) original occupants remained healthy. That kind of neglect can not be supported by a thin layer of gravel and a filter. The substrate in question here is EcoComplete to a depth of between 3 and 4". As OinKY said, it's bound to be chock full of microbes, but I wouldn't call it "live", more like "biologically active". Yeah, that's hair splitting. But when you're already accustomed to the terms "Live Rock" and "Live Sand" meaning substrates chock full of benthic organisms visible to the naked eye (most of which consume detritus so that you don't get the mulmy conditions I did), you kind of want to make the distinction. Microbes will never exist in large enough populations in a tank to keep detritus from accumulating, long term. "Live" substrates, in the sense that I am using it, can exist for decades without ever accumulating any detritus at all.