It's not decline, decline mean they have had made progress. In this case, if they are still using WW2 or WW1 style tactics, that mean The Russian Military did not made any progress since 1940
The problem as most military analyst see is the recent "Military reform' championed by Shoigu. Before this war, probably around 2010 (Can't remember when Shoigu were appointed Defence Minister, and don't bother to look up), Shoigu started making changes to Russian military deployment order, moving on from the tradition Regimental deployment that was used since cold war.
The change was supposedly reflect battlefield changes and give Russia a more "Organic" deployment, however, no other changes was adjusted to that effect, everything from staff management to logistic (Oh logistic) remain the same, using the same concept since cold war.
Smaller organic unit needs mobility and instant support to make it work, otherwise when you are getting pin with a smaller unit (as it happened in Battle of Kyiv) you are going to get bogged down. There are 2 major issues as we can see from the current war. 1.) not enough use of the Air Force 2.) The phaseline concept of Logistic simply cannot cope.
In smaller unit engagement, air support are primary, since you need to out manoeuvre your enemy, by putting troop and supporting element in place before your enemy did, the only way you can do so is by air, which would require a complete dependent of the Air Force. However, the old Soviet concept have set that Air Force is a supplementary element with its role relegated to supporting the ground movement. With that, Air Force don't fly SEAD, and take out Air Defence and the only role of the Russian Air Force is for Close Air Support. And without that air dominance, you don't get to deploy troop from A to B quicker than the Ukrainian did, especially Ukrainian have the home turf. That is why the Russian unit did make some initial gain but all but evaporated at the later stage of the first phase.
Just to make a major comparsion, according to US Air Force (which monitor Air Sortie in Ukraine) Russian launched some 300 air sorties during the first month in war. At the first Iraq war, US and Coalition launched over 100,000 in the entire air phase (also lasted roughly a month) of Gulf war, preceding he land incursion. You can't achieve air superiority with 300 sorties no matter how incompetent your enemy air defence is.........
Another issue Russian facing is the logistic can't cope with the current reform, as the concept of logistic has not undergo reform. in the old way, logistic is done by rail and logistic hub were litter around the rail line. That is due to the fact that since cold war, Russian Army move in bulk, deployed with regiments and you supply the troop in a regimental scale. However, this is not going to work when you break down the unit into battalion side and try to support each Battalion by land. Compare the US Concept of logistic, the log train goes from regional command (in Afghanistan for example, there are 4 regional command, RC North, South, East and West) and from those Regional Command, dedicated logistic line were use to feed troop forward deployed using medium lift helicopter and dedicated convoy network, say if I want to supply my troop in Kandahar region, first I fly my supplied to KAF then filter them out to each camp, and from then either fly them our of truck them into each FOB.
In Ukraine, this is not done, everything done thru central axis, so where your troop goes, your logistic goes, that's mean both log train and troop uses the same MSR to get into position, the heavy use of roadway plus being able to ambush would mean both troop and logistic would be stuck somewhere along the road simply because everyone is using the same road to get to where they want to go.....That's why you have that 60km convoy, it may look impressive but in realistic term, that is nothing but a giant target.
Which mean when they try their "BTG" concept, this is bound to fail, and when it does, Russia get no choice but to revert it back to how they used to fight, a giant fist depending on their artillery superiority that try to crush everything in its way, if you look at the current battle of donbas, you don't see Battalion Tactical group working individually fully taking advantage of their organic structure, but instead, you are seeing a giant blob of Russian force, that account for the slow progress in Donbas. In fact, it is lucky for the Russian, because if Ukraine have more artillery than they had or have a more competent air force, they can disrupt the Russian force by thinning out the herd.
