Conservatism has always been about policy and agenda.
Not Dear Leader populism."
Ronald Reagan's campaign themes in 1980 included: 
- The economy: "Are you better off than you were four years ago?"
- Carter's weakness: "The Time Is Now for Strong Leadership."
- Foreign Policy: "Peace Through Strength"
- Reduce Big Government: "Government is not the solution to our problem; government is the problem."
- General uplift: "Let's Make America Great Again"
In 1984, Reagan's campaign themes were:
- "Morning in America"
- "Bringing America Back"
- "Prouder, Stronger, and Better"
And it was quickly dropped and forgotten after the 1980 campaign.
By stark contrast, MAGA is Trump's major theme and purpose, carried forward through three presidential elections and their mid-terms over the past 10+ years.
It was not just one & done, it's been his slogan and focus from Day One.
So, MAGA absolutely belongs to Pres. Trump -- it's his to define and to accomplish.
Any future candidates & administrations claiming the MAGA mantle will be judged by how closely they follow Trump's definitions of what MAGA is & should be.
Yes, of course, if you wish to claim that MAGA is not truly conservative according to your definition of "conservative", that's an argument you can make -- you can easily claim, "we are the true conservatives" (if indeed you really are), but you cannot claim "we are the true MAGA", because that term belongs to Donald Trump -- it's his to define and to bestow on loyal supporters.
If you're not one, then you need your own slogan, it's can't be MAGA.
