You can definitely get by with just English here and have been for a while. In fact, when I first lived in Helsinki in 2000 you had to speak Finnish in some restaurants since the staff didn't speak English, whereas now it's the other way around.
There are however problems with English language primary education: while there are enough English language daycares, there's a shortage of (free) places in English language schools. Hopefully that's something that will be addressed soon, though.
If you're considering Finland as a place to relocate to, check out these links from our startup's blog:
https://www.toughbyte.com/blog/relocating-to-finland-as-a-de... https://www.toughbyte.com/blog/finnish-contract-developer-fa...
These links might also be interesting:
https://www.helsinkibusinesshub.fi/90-day-finn/ https://www.businessfinland.fi/en/do-business-with-finland/w...