I like Tilly but I think it's absurd to make her XO. If they somehow lacked a suitable candidate onboard, surely Starfleet would have a few candidates, especially considering the limited size of the fleet.
Put it this way...if Riker was killed in BoBW, what would we think about Picard promoting Wesley to first officer? Or Ensign Nog replacing Kira? Or Kim replacing Chuckles? Mayweather replacing T'Pol? All reasonably capable officers, but none of them ready to be second in command yet.
Exactly. All those jokes about 'Acting Captain' and 'Acting Chief Engineer' Wesley Crusher from early TNG will now be applied to Tilly. And they might overshadowed her character and talent, which I think are plentiful and still has lots of growth potential.