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Avionics can be and are regularly updated as needed. A lot of times they'll do things like that when a plane is in for a heavy maintenance check ("D check" in the airlines), where they basically take the plane apart, inspect everything, then put it back together. If it's in pieces all over the floor anyway, it's easier to upgrade the avionics while you're at it.

20 year old airframe is nothing. And given its age, the latest heavy maintenance could have been just a couple years prior (can't know without having access to the maintenance logs, obviously).

It's pretty routine to have an older airframe with updated electronics.

Also, radio altimeters, GPWS, weather radar, etc. have been on commercial planes for longer than you might think. The main changes are going to be in the electronics and the software, not the hardware.

In the specific case of PLF101, it was a Tupolev Tu-154M. The oldest of those variants started delivery in 1998, meaning the aircraft was at most 12 years old at the time. The M variant also had upgrades to include Western systems for the FMC (Flight Management Computer), GPS, EGPWS (Enhanced Ground Proximity Warning System), TCAS (Traffic alert and Collision Avoidance System), etc. So in all respects it was a pretty modern aircraft for the time.

Both Russian AND Polish authorities found no fault with the aircraft, and concluded the crash was caused by unsafe operations for the weather conditions encountered. The Polish Air Force unit operating the flight was actually disbanded over this when the investigation uncovered serious deficiencies in organization and training.

tl;dr - Yes, it was pilot error. There's always the slight chance it was intentional "error", but given that the entire unit was a shit-show, it seems more convenient than anything. Unless they intentionally put the biggest retards up front hoping the weather got the best of them.