If they let Derozan walk because he's not worth what he's asking, then he's easily replaceable, and you don't have to replace him with a guy who will "catapult you into contention" because that's not what he was/is. If he's not easily replaceable because he's improved so much this season and you want him as part of your core, then he's worth paying what he's worth. Pretty simple. If everyone at the same talent level of Demar is asking the same amount of money, it's not overpaying, it's paying market value, which you allude to yourself.
2. It absolutely matters if the Raps get nothing for DeRozan, because there's very little chance they can replace him with a player that will catapult the team into contention. As for overpaying, consider the lack of quality 2 guards in the league, the fact that market value in the NBA is tied closely to offensive production, and that there are a lot of teams out there who have a lot more money than NBA talent on their rosters. I can easily see someone offering DD 9-10 mil per year based on untapped talent, especially if he plays well this year.
No one is reading into your post. You said he would walk if Toronto isn't good enough. Which is both as unlikely as it is plausible. Or, in other words, no one knows what he would do if Toronto isn't a playoff team at the end of next season. But if that's the case, Lowry may not be worth retaining, either. If he can't help take this team to the playoffs in the next two years, he shouldn't be considered for a huge raise and may even be expendable -- along with a lot of other guys.
3. As for guaranteeing that Lowry will walk, I can't obviously - you're reading too much into a hastilly written post. But the only leverage the Raps have is an extra year on his next deal, and unless they're going to pony up a max contract, that hardly matters because another team can outbid them. So to me, the only way lowry stays is if the raps overpay or if the team shows enough growth that he wants to be a part of it going forward. As a guy who's all about winning, I would suspect he'd choose to partner with a real franchise player in pursuit of a championship over sticking with a treadmill team for another 4-5 years of first round playoff exits. But he might surprise me.