It was in Putin's (and Russia's) long term interests to drag out the conflict as they have, primarily for two reasons:

1) By setting the Russian economy on a war footing and out-producing NATO by every measure available, Russia was able to prosecute a years' long war that not only destroyed the Ukrainian armies (three of them so far) but also served to completely demilitarise NATO - or at least those NATO members in the European theatre.

2) Putin understands that he faces an existential threat in NATO and the West, and that they will not stop until either they are defeated or Russia is defeated. The strategy of a long, drawn out war of attrition serves not only to drain the West's resources for waging war, but allows time for Putin and Lavrov to manoeuvre globally, ushering in the "multipolar world order" that will spell the downfall of Western hegemony (e.g., BRICS) .

And Putin has been successful: look at the cracks that are already developing in NATO; look at the leaders that are being overthrown. By keeping the sanctions in place, Europe has destroyed its economy, and the EU itself is falling apart at the seams. And meanwhile, BRICS expands at an ever faster pace.

I explained it all in this piece that I wrote last February:

"How Putin is WINNING in Ukraine"

https://euroyankeeblog.medium.com/how-putin-is-winning-in-ukraine-8809510528c1