This is how every club in the world operates though? Every team loses a key player eventually, either through age or them leaving .. and they manage. We've replaced Eriksen's presence in midfield with a top class young talent in Lo Celso and one of the most coveted young central midfielders in the world in Ndombele. Sure, the chance is always there they aren't good enough, but with both of them coming in we should see a net improvement in our midfield play than if we just had Eriksen amongst shite.
And the sale price of a player is dependent on a number of factors, not just how big a talent they are. Other teams are pretty well stocked when it comes to attacking/creative midfielders, so there wasn't huge competition from mega clubs for his signature. Betis originally wanted 70mish which is still big money for a young talent (what was frenkie de jong? 75m euros?) but clearly the player pushed hard for a move all summer so they dropped it somewhat, but still to a price which would suggest they rate him extremely highly as an asset. They're hardly a huge club who can resist big money offers and refuse to accept unless the fee is tremendously high.
Regardless of the Eriksen sale, I believe that we've improved in the midfield area this season. Winks/Ndombele/Lo Celso is a really well balanced midfield, and again we don't need Lo Celso to be as good as eriksen (although, from what I've read about the player from La Liga journalists/people who have consistently watched him, he is rated extremely highly) merely to come in and be a successful signing, and we can build in other areas so we become a better all round side.
Every team has to deal with the occasional big player moving, it's not a huge issue at all as long as we do go out and replace his presence in the team. I think we've got about that the right way with these signings and I hope we also do it for other positions.