Why combine stats, especially the 40-yard dash, relevant and referred to years after the draft?

it would surely be possible to get more up-to-date measurements of running speed and/or acceleration one would think

Absolutely it's possible, and in these days of analytics every single NFL team will be doing this for every player multiple times a season. But they don't make that data public, both because it could give an advantage to opposing teams and because the players don't want it made public.

Pretty much the only things which are public are the player's combine performances, so they get used.

one would think it would surely be possible to get more up-to-date measurements of running speed and/or acceleration.

Yeah there are more up to date stats on players stats and acceleration, but not official results captured at the same time where all teams agree on them. A lot of times when watching games, if a receiver or punt returner breaks out defenders with speed for a score the broadcast shows their speed throughout their run just seconds after it happens. So we definitely have the technology for it. These measurements are also in game, with pads on, so you could argue they're meaningful than a 40 yard dash in the combine.

The point of the dash is that almost everyone who gets drafted goes through the combine and does the 40. So you have a stat line of those who got drafted on that year doing the same run, within days of each other under the exact same conditions. This is a stat line the measures a player's baseline speed/acceleration of when they got drafter. And since majority of draftees do it, it gives a precise measurable to the GMs to compare guys's speed/acceleration at the draft time.