As you may know, the problem with the Paschalion is that it is based on an ecumenical council's decision (Second?) to fix the Vernal Equinox on March 21st. That date corresponded nicely with the actual Vernal Equinox at that time, which is important because of the formula: Pascha to be celebrated on the fist Sunday after the first fool moon on or after the Vernal Equinox." Now, if the civic calendar starts to deviate from reality, one has to choose between (a) bringing the calendar up to date (as the Gregorian and Revised Julian have done) or (b) pretend that the old calendar still reflects reality. It so happens that March 21st falls on that date on both Gregorian and Revised Julian Calendars, but it "happens" 13 days later on the Julian Calendar. So, there are times when we can all celebrate Pascha at the same date (that is when the first moon is on or after March 21 +13 days), but not often.
To reiterate, if every Church used either the Revised Julian or the Gregorian calendars for both the Menologion and the Paschalion, we would all celebrate all of the appointed days of the liturgical calendar at the same time. At least in our and our great-great-great-great childrens' time for these two are bound to diverge sometime in the far future. Whereas, the real possibility exists that at that point in time, Nativity will be celebrated by the Old Calendar folks even further than the date on the Church liturgical calendar, for the Julian gets worse three days every four centuries.
Everything Carl wrote is true, but it is only part of the story. The whole story is even worse.
The Solar drift in the Julian calendar is about three days in 400 years, as Carl noted. But in addition, the lunar part of the Julian calendar (the paschalion) also has a drift. The paschalion's formal lunar phase falls behind the average lunar phase at a rate of about 1 day in 300 years. The error has accumulated to where, according to the Julian paschalion, a full moon now looks like this:

So that now, Julian Easter is not the first Sunday after the first full moon after the vernal equinox, it is (approximately) the first Sunday after the 4-day waning gibbous moon after the first full moon on or after March 30th. When Eastern and Western Easter are only a week apart, it is due to this lunar discrepancy, not to the discrepancy in the equinox.
The Milankovitch paschalion, if implemented, would compute the moon's motion more accurately than the Gregorian, which uses the average lunar motion. The price would be more complex arithmetic for a resulting date of Easter that differs from the Gregorian only occasionally, as it will in 2019 and on a handful of other occasions in the 21st century.