Don't know why we're comparing them in the first place given one is a fullback and the other is a striker but seeing as we are, we can't use the lowest figure for walker and the highest figure with add on's for Lukaku to make it sound better. The fee going around is £55 million + add on's and seeing as we don't know how much the add on's are worth we can only compare it to the £75 million base figure for Lukaku.
as for what we're getting, I have no issues with most of them but 3 of those points are very questionable. Firstly, I wouldn't class him as young seeing as he's 27 and fullbacks tend to drop off quite early, particularly ones that rely so heavily on pace as walker does.
Secondly, when has he ever played DM? there's nothing in his game that suggests he could play there either, lacks the technical skill, positional sense, brains and composure to play there and under utilises his main asset which is his pace. I'll put money on him never ever playing in that position for us.
And the final point, does he really weaken them that much? If he did they simply wouldn't sell him. They obviously prefer Trippier (as do i personally) and feel he's the better bet going forward, so losing a replacement right back isn't weakening the squad all that much, particularly when you're getting £55 million + for him in a time when money might be tight for them with the new stadium to pay for.