Remember most of the front is angled armour and then engine and then crew compartment and troop compartment under the turret...
Still, isnt that just what they have done? Orderd a small batch and stated that they will continue with upgrading T-72? Further massproducing was not in the pipeline?
The Armata is not a new tank... it is a new vehicle family... including tanks and BMPs and BTRs and everything else in an armoured division.
They need to test them to make sure they are right before putting them into mass production...
You get it right before you make lots of them.
With the T-72 they are not going to "make" any... the T-72s are already made and will receive upgrades depending upon how many they want or need.
The T-90 will be produced in numbers depending on what results they get with the Armata and Kurganets and Boomerang tanks... they might make enormous numbers instead of enormous numbers of the newer vehicles if they need more work, or they might only make a few hundred to fill gaps while the new vehicles are mass produced.... or they might make more so they have time to fix the problems with the new vehicle families.
Once the new vehicle families are sorted out an enormous number of support vehicles will become obsolete and will be replaced.
The light and medium vehicle families will be likely produced the quickest and cheapest, but armata vehicles will be more expensive and more specialised and likely will take rather longer to get all the vehicles into service... most T-90s will probably go into the reserve while T-72 upgrades will be used as robots.
A well protected T-72 whose crew wont bail out at a penetration... fire suppression systems can be very effective when the internal parts of the hull and turret are nitrogen filled with no oxygen for fires to burn... it can roll into an enemy held area with APS and ERA protecting it from most hits, while UAVs and other tanks are looking at where the enemy fire is coming from and suppressing it... the robot tank itself can also engage targets.
Of course against a sophisticated enemy some sort of signal jamming equipment could be used but that could be detected and targeted from the air too...
We really don't know how future warfare will play out... they want armour for a full division to test what they were thinking when they came up with the designs and to progress to further ideas... since they designed armata and boomerang and kurganets they have been to Syria, so they might have learned things there too... their armour needs to be flexible and be useful in a range of situations from COIN to WWIII.
Mobility and firepower and protection... the three keys to armour... boomerang has high mobility and high firepower and good protection... kurganets has good mobility and high fire power and rather good protection (remember APS that stops APFSDS rounds and HEAT rounds plus normal armour and ERA...) Kurganets is probably better armoured than any current armoured division... a T-90 tank might have slightly better protection from the front, but even in a tank division only a small fraction of vehicles are actually tanks... most are BMP or BTR vehicles with much lower levels of protection...
And of course the armata with good mobility, but high fire power and high protection... most divisions have nothing like that level of protection... but in terms of fire power and situational awareness... the armata tank is supposed to have a tethered UAV with radar and thermal sensors that can hover over the tank and provide a birds eye view to the tank and other vehicles nearby.
These new vehicles have much better communications equipment and will share information and provide battle management systems with data to make them useful... other platforms including recce and attack formations will further provide live info in combat...
Previously at sea only large ships got such systems... AEGIS is an example. The new Sigma system in the Russian navy spreads that system to all platforms to maximise the data detection and data collection and data sharing capacity to make it much more powerful and these new vehicle families will do the same for land vehicles...