What will you do if some altcoin (XMR or ETH for example surpasses the value of BTC, either in market cap or by the price of a token each?
there already are a couple of altcoins with higher price than bitcoin there are already other altcoins with bigger market cap than bitcoin too!
Will you sell your BTC to buy the more "valuable" coin,
being "valuable" is not defined by price nor by market cap.
the value is the coin itself, the network, the wallets, the security of the code, the hashrate, the services, merchants, users,...
Also, what do you think are the chances that it could happen?
it has already happened so the chance is 100%

it is just price, not "value" though
here is the list in case you were wondering:
coin name ....... price
Bit20 $199,075 (~200K)
Project-X $165,012
42-coin $8509.41
Primalbase Token $4566.06
bitBTC $4540.86
Bit20 has $0 Trading Volume (24h)
Project-X has $236 Trading Volume (24h)
42-coin - $601 Trading Volume (24h)
Primalbase Token - $6,397 Trading Volume (24h) which is better than of the above ones but still is similar to the Trading Volume of the worst coins around.
bitBTC has $78,911 Trading Volume (24h) which is also too low to be taken seriously.
As far as I know, the only coin that surpassed Bitcoin in price and at the same time had a decent trading volume, was Zcash. But that lasted only 5 days in the very beginning of the coin's existence and never happened again.