I'm not sure why you feel that way. Willie is a proven NBA player, who may be on the verge of a break out year (maybe). Whomever we pick, will have some adjusting to do. None of them are going to waltz in and dominate at the center position. Lets say we pick Ayton. Why not move Willie to the PF position where he has traditionally put up better numbers. People forget that's the position he played at Kentucky when he was paired with Towns. Or, you could move Willie to the backup center position and let Giles take over the PF position. After next year, unless he's resigned, Koufos is gone. I still don't have confidence in Skal.
So at the end of the day we could be left with just Ayton, Willie, and Giles as our big's. So I don't see why it becomes necessary to trade Willie just because we draft another big man. I might add, that Willie is exactly the type of big man that everyone in the league is looking for. Athletic, runs the floor and can defend on the perimeter. Willie isn't a great rebounder, but he did average 7 a game last season, and as I pointed out, only 11 NBA players averaged double digit rebounds last season.
I've been meaning to do this for a while, but haven't got around to it. I don't think lot of playoff teams are playing 3 bigs big or meaningful minutes. In some cases, it is the case of roster construction. Teams like Rockets, Warriors, Blazers, Wizards have their stars and money on players at the guard or wing positions. So bigs (with the possible exception of Capella with Rockets) have limited role there. Even teams like Cavs, that give good amount of minutes to players like Love and Korver, treat them more as perimeter players.
Apart from Pelicans, who had two major stars in Boogie and Davis, I can't think of any teams that have two significant assets at 4 and 5 (maybe Utah to some extent), and virtually no team has 3 (AFAIK).
Is it too premature of us to talk of 3 when we don't even have one proven one so far? Certainly yes. That said, the purpose of the draft is to project the team 3-4 years (and beyond) down the line, and not just next year. If we draft a big, and if the hype around Giles is real, we have two major assets up front. Can they become stars, stay healthy (for Giles, these questions will keep coming up until he proves otherwise), play with each other, help the team win, and hundred other questions will be answered over time. However, if the FO thinks they can, then we don't have space for a 3rd big, who is a major piece. It can be some kind of specialist who fills specific needs (most likely some kind of enforcer or a 3 pt shooter). I think Willie will be too good and expensive for that role, nor is he likely to be pleased with it.
This is one reason I'm not too keen to draft a big if Luka is available, unless the FO is convinced that the big is much better. If so, then you get him, and get SF through some other means. Else, if you think the big is better (say Bagley for this discussion), but not by much, you have to see if the combination of Willie/Luka is better, or that of Bagley/what we get for Willie.