To the contrary, I have no doubt that Fidelity is excited about this move by Vanguard. More money will be coming into their platform (especially their higher cost money market funds) with minimal incremental expense. I am only a sample of one, but at my average annual balance in Fidelity's lowest cost investor money market fund (FZDXX) (equivalent to Vanguard Prime Money Market account), they will be making $450 annually on that alone. The costs of servicing my cash management account has to be a fraction of that and I would be one of the lower margin customers at Fidelity because I intend to keep my cash management account at $0 and use overdraft from FZDXX. There may be other customers who stay or keep balances in lower yielding options (cash management or higher cost money market funds).
That does not include the IRA rollovers that will be coming over and new table investments make from the brokerage account that were at or being invested at Vanguard. Fidelity may not make much on those as they are primarily going into their index funds or iShares but it is incremental revenue nonetheless at a high gross margin. I am keeping taxable assets at Vanguard in part because of the trapped unrealized gain and in part because of Vanguard's superior muni fund options (especially compared to Fidelity or ETF equivalents). However, there is not doubt that over the next decade Fidelity is likely to make thousands of dollars (if not more) off of me at Vanguard's expense.