I don't believe that having language included excluding the use of another franchise tag is an unreasonable ask by Prescott and his people. Nor is asking for the inclusion of a no trade clause as well.
If he is "your" guy, commit to him. They have played their hand regarding him since his rookie year. So, as long as the numbers are reasonable enough to still have room to field a talented team around him, and the years are agreeable enabling the team to spread his cap hit...add the language and get it over with. If he isn't your guy...tag and trade him and move on.
As an aside, knowing he wants this language leads me to presume that Prescott won't budge on wanting a shorter deal so he can test the market again sooner rather than later.
On the other hand, how many times have we seen a team commit to a guy with a huge contract, only to find out he isn't what they thought, and end up wanting to dump his contract?
I would say contract bust happens more frequently than contract success.
If Dak is confident that he is the man, why demand the anti-tag and anti-trade clause?
Play well, and take the team to a few playoff wins and you'll probably be a Cowboys for the next 9-10 years, and make enough money to last you ten life times.
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In this case I understand both sides. Dak wants the big contract with all the safety nets. Cowboys don't want to commit huge money, and contract, on someone who may, or may not be, "THE MAN".
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It's not my money, so I can't say whether cowboys should [totally] commit or not.
The poor guy, if he is tagged, he'll only make about $38,000,000.00.