First off, 8 years is actually historically a pretty rare term for high-end players coming off of their ELC; even more-so for players of Matthews' caliber. There was a bit of a trend starting to form a few years ago, largely due to those specific teams being able to afford overpayments at the time for extra years, but that was not a common thing, and it died out pretty fast as available league-wide cap space shrunk.
Second, both of those players signed years prior. Their cap hits under the current cap would be 13.6m for McDavid and 10.9m for Eichel.
Third, we know for a fact that McDavid's contract does not reflect his market value, because aside from not using his full leverage in negotiations, he very publicly took a 750k discount after the already agreed upon price was decided. That means that his negotiated contract value under the current cap would be 14.4m.
Fourth, even without that public discount, McDavid would have and does have one of the best contracts in the history of the cap era, so he's not really a good measure for evaluating how fair a contract is.
Fifth, you are evaluating Eichel based on (an inflated perception of) what he is now (which is still worse than Matthews), years after signing, at an older age than Matthews. At the time of signing, Eichel's contract was one of the worst post-ELC contracts. That was a contract based on hope and trust that Eichel would improve. Luckily, he did, but those 8 years required a massive overpayment at the time.
Even if we ignore the significant disadvantages Matthews faced in opportunity (2nd unit split PP) and linemates, at time of signing:
ES P/60
McDavid: 2.98
Matthews: 2.78
Eichel: 1.78
ES P1/60
Matthews: 2.40
McDavid: 2.32
Eichel: 1.44
ES G/60
Matthews: 1.59
McDavid: 1.09
Eichel: 0.79
PP P/60
Matthews: 6.45
McDavid: 6.42
Eichel: 6.29
PP P1/60
Matthews: 5.05
Eichel: 4.75
McDavid: 3.60
PP G/60
Matthews: 2.94
Eichel: 2.52
McDavid: 0.94
To help visualize, based on these pre-signing levels of production, if we equalized everybody to a standard (16:30 ES, 3:30 PP), production over an 82 game season would look something like this:
Points:
McDavid: 97.9
Matthews: 93.5
Eichel: 70.2
Primary Points:
Matthews: 78.3
McDavid: 69.5
Eichel: 55.2
Goals:
Matthews: 49.9
Eichel: 29.9
McDavid: 29.1
Also, another note; 26 is not "just entering your prime", and there's no evidence that Matthews wants to go anywhere else. McDavid and Eichel aren't really the best comparables for Matthews, but even when compared to them at time of signing, Matthews' contract does not look bad at all.