leaning towards spym > voo... but is there a risk spym stops tracking the s&p 500 eventually?

opening a brokerage through fidelity. wanting to invest in the s&p 500. as much as i want to go with vanguard etfs after reading the simple path to wealth & the little book of common sense investing... spym has them beat on the expense ratio (0.02% vs. 0.03%). (i know fidelity offers fxaix at 0.015%, but i think i want flexibility to transfer away from fidelity if i need to down the line.) anyways - i've read that spym (formerly splg) used to track the russell 1000 until 2020 or so, and then swi