Good points, Gun. Tanks have two basic roles in war:
1. Support the Infantry by breaching fortifications. A tank in this role must be able to:
a. go where the infantry go
b. charge over a trench
c. smash through concertina wire and other obstacles
d. demolish bunkers with direct fire
e. gun down enemy infantry
f. survive fire from Infantry weapons.
2. Anti-tank. A tank in this role must be able to:
a. outmaneuver enemy armor
b. effectively destroy enemy armor with its armament
c. survive fire from enemy armor
The problem is that these two roles conflict. The first role demands a small, light vehicle with numerous smaller weapons (MGs, AGLs, demolition guns), i.e. a light tank. The second demands a heavily armored behemoth with a few powerful weapons (high velocity gun, ATGMs), i.e. a MBT.
The AMX13SM1 is not that bad a tank in the Infantry support role. It is small and powerful enough to squeeze and bash through jungle terrain and be where the infantry are. Its gun is good enough to demolish most field fortifications. It can be fielded in enough numbers to lay down lethal anti-personnel fire with its MGs. Its front armor is thick enough to face enemy infantry. I wouldn't mind having one of these with my platoon in the middle of the jungle.
However, it really really really sucks in the anti-tank role. It is far too lightly armored and its gun is deperately inadequate. It is so bad that it is a joke.
The Infantry support role is more important for combat in our region, due to the terrain and the fact that potential opponents don't possess much armor. Also, the best anti-armor platform is the helicopter, rather than other tanks, even MBTs (and we have Apaches now). However, our choice of light tank should not be SO WEAK against enemy armor.
As long as the new light tank does not sacrifice too much in pursuit of the anti-tank role, it will be good. It needs to be just as powerful and small as AMX13SM1, so it can climb hills, cross streams and squeeze through jungle. It should still have multiple MGs and AGLs, not just a coaxial MG. Whether this can be accomplished while giving it heavier armor and a new 105mm gun is the question.
The Bionix chassis is pretty good for this. It is very compact, in particular very narrow. It's only 20cm (8%) wider than AMX13SM1, though it is 50% heavier! I think that skinny-ness was one of the main design criteria for the Bionix since it stands out so much. The Bionix currently has the same power/weight ratio as the AMX13SM1 but a tank gun will reduce it, so a beefier powerpack will be needed to keep it at 20 hp/ton. The armor protection is already sufficient for a light tank role.
On a completely different note, the best light tank in Vietnam IMHO was probably the M50 Ontos. It had six (!) 106mm RRs, four .50 cal spotting rifles and one 7.62 mm MG. It proved to be the tank that the VC/NVA feared the most. Look at
http://ontos.homestead.com/ms3.html