Suggest you listen to xcpa and invest elsewhere if you don't like the MMP/MGG arrangement.
Second, why do you think MMP issues new units? (PS- it has no shares outstanding, only units) It issues units to acquire assets that hopefully will provide additional DCF for the new larger number of units. The value of your units is not diminished because of the value of the new property.
Last, if the cash flow did go down with the increased number of units the GP incentive payment would go down as well. If the distribution rate were cut to 50% of today's rate ($1.10 unit) the GP incentive amount would be cut from its current $47.4M annually to zero. Suggest you do a bit of reading. The recent MGG filing with the SEC or the last MMP 10-K could help you better understand the companies.