It is pretty much the whole point. Dez wants the most he can get with injury protections whether it would hurt the team or not. Dallas loves Dez but they have to think of the whole team.The sticking point is the guaranteed money. Why should Dallas assume all the risk? There are other contract options and other players available. If Dez won't share any of the risk Dallas will tag him , maybe twice and move on.
You are correct on all accounts.
The team is under no obligation to pay Dez a long term contract if they do not want to. They can simply tag him twice (the max allowed under the CBA) to retain his services.
Likewise, the player is also under no obligation to agree to a long term contract if the team is unwilling to meet his demands. If Dez feels he should be the highest paid WR in the game, then he has 2 seasons under the tag to prove it. After that, Dallas cannot stop him from going to another team that will pay him what he thinks he deserves.
I know the saying gets old, but this really is a business and both sides are looking out for their own best interest when it comes to contracts. Dallas wants to limit the guaranteed money so if Dez gets hurt or if his production declines, they can get out of the contract. Likewise, Dez wants to maximize his earning potential while he can. As a player, the next hit may be your last so you never take anything for granted.
A contract will be agreed to when Dallas feels they are protected in terms of guaranteed money and Dez feels he is protected in case of a severe injury. The hard part is compromising to get to the magical number that both sides can live with.