Downing will be 27 before the season starts, he's not likely to be playing at a high level for much more than 5 seasons. As for Henderson, he's played regular Premier League football for the past 2 seasons; that's fairly established.
You can pay £15m of whatever he costs on Downing and you'll have to replace him in 4-5 seasons. You can pay £16m on Henderson and won't need to replace him for 10+ seasons.
If you can name me a player in the current Liverpool squad, signed more than 10 years ago as a proof that this is the ethos of the club, I'll be mightily impressed.