If you can afford and you want, the only argument I can put forward is less ewaste if you give a second life to the many very decent professional thinkpads that are retired every year. My employer is now going for a 5 year renewal cycle, used to be 3 for a long time. Unfortunately I couldn't even buy back mine when it expired because it is a lease subcontract. It had an i5 7th gen and 32gb ram, was buttery smooth even running windows and I dreamt of running Linux on these.

Depends on what you call 'old' and what your use case is. My T495 was less than 300€ and it does everything I need from a laptop easily. Bigger drive would be nice, but once the summer is over I rarely need to pull 4K video from sd-cards in temporary storage, so I doubt I'll bother to upgrade it any time soon.

I wouldn't say that, maybe in the case of the x31 or similarly really old laptops, however newer old laptops like the t60p or t500 aren't that bad and can still handle every office and internet related workload just fine