That is a very narrow example you are making. But to your point, if a team thinks they have a contender at every position except at QB, then absolutely a big team breaks the NIL bank to nab Grier. Just look at how the NFL operates in that regard. I also think a player like Grier is tempted to go regardless of a better NIL deal if the opportunity to be on a national championship contender exists. So WVU is behind the 8 ball in that both teams could outbid WVU with NIL money or even offer a bit less NIL money in exchange for a higher level of possible achievement as a way of out competing WVU. WVU is assuredly sunk if a team can offer a high percentage CFB playoff contender AND more NIL money.
There are also two things your hypothetical leaves out. First, how about the rest of WVU's 2018 roster when Grier and Sills were seniors? Does WVU keep Yodney Cajuste and Jennings as well? Both of those guys could've been the top at their positions for several lateral teams (in terms of program size, but bigger in terms of community money) like UNC, NC State, etc... You think the WVU community ponies up the money to keep all the parts that were necessary for that teams success? How about Kenny Bigelow, Josh Sills, Marcus Simms, David Long, Kenny Robinson, or Askew-Henry?
Second, how does WVU keep a Will Grier and David Sills long enough to have the chemistry year you are talking about? You think if either of them play at a good level as redshirt/true freshmen and sophomores that they won't have others in their ear? Maybe there is a core group of players that don't look good enough to tempt schools with bigger prestige and NILs than WVU that have a revelation all at once and all in their Junior or Senior year for a magical season at WVU. But that is highly unlikely.
I am not defending Brown by any stretch. That being said, if you think that the NIL and an instant transfer process doesn't significantly hurt WVU's chances of building teams with an outside shot at the NC compared to before you are blind to the truth.