Trading Theis, Romeo and a #1 for a guy who would be angry about being a 4th option seems risky.
This is a risk for sure, and I do really like Theis as a player. That being said he probably is the 5th best player in our starting lineup right now, and certainly is not irreplaceable - I'd put him in a similar category to Jae Crowder when we traded him out for Kyrie. Good player, but replacable and probably worth the risk if the is a high upside return.
Romeo I don't really care for in the slightest. He spent most of last season injured, and when I have seen him play I've seen very little from him to really excite me. Way I see it he's entirely unproven at this stage - a 50/50 propect who could go either way. I have no issues dumping him if it means upgrading our team.
A first round pick I also don't really care that much about in the grand scheme of things. I doubt the Celtics are drafting a game changing core player with any of their current draft picks, so to me that pick is most valuable as a trade asset - which is what we'd be using it as here.
Hield is a standout shooter and a potential perennial 20 PPG scorer, who also happens to line up very well age-wise with our current young core of Brown/Smart/Tatum. If things do go south with Kemba's health then Smart potentially becomes our starting PG and Hield could potentially become a #2 or #3 option for us on offence. With Brown, Tatum and Smart in the starting 5, a top shelf shooter like Hield could be exactly the type of piece that could help stretch the floor and take this team to another level. He could be to Tatum and Brown what Ray Allen was to Piece/KG.
It's a risk, but I'd happily take the chance if that's all we need to give up.