2. She's Fought Bigger
When it comes to the Avengers, the most powerful member on the team is Thor. He's fought mythical creatures his entire life, and is the one Avenger who comes tantalisingly close to killing Thanos dead in the film's finale. It stands to reason, then, that the God of Thunder poses the deadliest threat to the Mad Titan, and while that's true for now, underestimating Captain Marvel would be a huge mistake.
Confident, self-assured and replete with the move-set to back it up, Carol Danvers has tangled with the best the Marvel Universe has to offer. She's single-handedly averted entire invasions, partially resolved the Kang Dynasty storyline by killing the Master of the World, and has genuinely been everywhere in every major Marvel event of the last two decades - usually to affect decisive change.
Make no mistake, Thanos is still undoubtedly the most powerful villain the MCU has - and likely will for some time - ever seen. Despite this, there is precedent for Captain Marvel to intervene and truly upset the status-quo - a talent she's perfected for much of her tenure during the 21st Century.
It won't be an easy fight obviously, but if the Avengers are to stand a chance against Thanos toe-to-toe, they're going to need all the help they can get.