IDK. "Holding the line" scenarios in last 100 years generally ended up badly for the defenders: expensive and ineffective. Modern wars are all about movement, maneuver and sudden concentration of power in a spot chosen by the attacker.

If the recent Nagorno Karabakh war is of any use as an example, Russians and Russian-trained officers were unable to hold their positions against killer drones. Drones are cheap and their destruction in combat does not fuel anti-war sentiments at home, unlike a stream of coffins draped in flags.

But given that Russians are not stupid, they are probably working on some anti-drone strategy as well.

Saying this, it's not a literal line of course, but a meaningful strategic objective, and rough position for offensive staging.

Yes, yes, yes. Long lines on warfare are an invitation for a breakthrough by a more concentrated force, which is what every Russian military textbook tells.

The Anti-Drone strategy in Nagorno Karabakh would have been airstrikes. But the rules of engagement were such that this was not doable.

Beyond that, actual Russia support was very limited as punishment for Armenia's reorientation. If this was a full-scale war then the skies would have been full of Su-35s swatting out the TB2s and killing drone operators.

The key to small drone efficiency is that they don't need big airfields which can be bombed.

Instead the attacking force will be hard pressed finding hundred separate drone teams hiding in bushes, and forests on the frontline, while flying over enemy AA themselves.

There will be no enemy AA. There will be no enemy air force. You do not understand the dynamics of this conflict. Ukrainian AA is entirely inadequate and it's airforce is a joke to Russia.

Beyond that, these drones are satellite-guided. You can't control them from bushes and forests. The Bayraktar TB2 that did the vast majority of the damage in Azerbaijan needs to take-off from an actual airfield, too.

What you might be referring to are loitering munitions. Their issue is that they have pretty dumb guidance, so all they are good at is going to predetermined GPS coordinate or homing on radar. The solution is to overrun their positions and use your airforce to shoot them down if they do take-off. They are very slow. It can be expensive to shoot them down but you only have to do so until you overrun their 60km or so range, and for the things in the danger zone you only have to use mobile assets that cannot be targeted via GPS.

In small numbers and without actual drones to help with jamming these loitering munitions are also quite vulnerable to AAA.

There are some long-range loitering munitions, like the IAI Harop. However they can only do radar-seeking missions without a base station and actually do need a base. Additionally munitions like the Harop have a very large radar cross section from anything but the frontal aspect and are thus very vulnerable to detection by modern aircraft radar.

TB2 on sale has no satellite link.

TB2 Video is line of sight gigahertz link. Control, and telemetry UHF which can get to 100km.

TB2 can launch from improvised strips with substantial payloads.

A mobile catapult launch adapter was once advertised

Some TB2 in Azerbaijan were operated by Turkey essentially, but I take your point.

TB2 may be able to launch from improvised positions. But it cannot land and resupply there. The weak point still exists.

The elephant in the room is also that the TB2s will get shot down by the Russian airforce. In Azerbaijan this did not happen because Turkey was willing to protect Azerbaijani airspace and the Armenian airforce is weak. In Ukraine the roles are reversed.