Reason I ask, is because Now I'm stuck with a battery that is very very dead... from that lantern I obtained from the pawn shop... apparently you cannot store them fully discharged. Why would they think to use a lead acid battery in a device which could be forgotten??

I need to get a new NP3-6 6 volt 3.0 Ah battery for it...
https://www.lighting-gallery.net/gallery/displayimage.php?album=4650&pos=0&pid=146566

Also could I perhaps easily retrofit another type of battery in it's place if need be? it'd need to fit in the same compartment...

Simple main reason:
Cost per stored Wh for the given size range.
Plus it allows float charging without causing degradation (vs LiIon - these tend to degrade the fastest when stored at full charge; assume voltage regulated charger).

The fact the inner workings are toxic is not that big problem when handled properly (Pb batteries are routinely >99% recycled for more than half of a century, just because making batteries from the recovered material is way cheaper than refining new one)

Lead acid batteries are inexpensive and have low internal resistance so they can drive larger loads than primary cells (like alkaline or carbon zinc). Biggest downside is that buyers don't always remember to keep them charged. Additionally, when the battery fails, I bet most people (or at least some people) will just go out and buy a new lantern (thereby meaning more revenue for the lantern manufacturer).

Lead acid batteries are popular for inexpensive "Million Candlepower" spotlights sold here in the USA. The batteries and charging circuitry is so poor you're lucky to get six months of service from the lantern.

I have a Streamlight "HID Litebox" that is now about 5 yrs old and haven't replaced its LA battery yet.I do NOT leave the battery on the charger-there are Liteboxes here at work where they are left on charge when not being used-the batteries in these have to be replaced a few times per year.Charge the battery after you use the lamp-then leave it OFF the charger until you use it again.The battery will last longer!

Adding some IT experience here

In UPSes the battery is all the time in charge mode

In cheap offline/interactive UPSes, the battery fails within a year or two even though it is kept charged and doing nothing

In online UPSs (battery connected to constant voltage charge) or atleast top grade interactive UPSs, the battery can last its full 5 years design life and more

The battery will last the longest, if it is kept on a properly voltage regulated float charger.
Something like this uses to be installed in things like quality UPS or so,
but even in expensive lanterns use to be nothing more than just a power resistor and a diode.
That does not prevent from overcharging.
The SLAs use to use the same trick as Nixx to recover the water back from the overcharging, but on SLAs it is way less efficient in that, so the cells are still loosing water.

This makes them hard to manage: Just storing them means they selfdischarge and then the PbSO4 crystalizes (so capacity loss), or when you periodically put it back on the charger, they will be overcharging to some extend, so loose water (again capacity loss)

Some years ago I installed charge management circuit (bulk charging till 7.4 and once full charge detected, switch over to 6.8V; first very simple with two transistors and a TL431, later replaced with this, fast charging capable one with the same functionality except higher bulk charge current), after that the battery lasted more than 5 years, which is the expected life of the basic SLA I've used there.

UPS units here at work have to be replaced all of the time from battery failure.Instead of replacing the batteries-the unit is replaced.There is a new UPS in our computer-TELCO room now.Replaced a larger one that was used with the old TELCO system.The new Avaya phone system uses lass power than the old one-and its in a smaller rack cabinet.

My experience with UPSs is mixed. Some units (even from the big name manufacturers) seem to cook the batteries by charging them at too high a voltage. As a result, batteries only last 2-3 years. Other units can keep the same batteries for at least 5-7 years with no trouble. It seems like older units are better in this regard. I have a 1980s Topaz that is the best on batteries of any UPS I've ever seen.