@Timo^ The E came soon after the D and the Defiant also followed its predecessor immediately.
Ultimately, it's all in one's personal continuity or belief and there's nothing to prove anyone right or wrong. But I don't find the version you suggest convincing. I'm sure the reverse is true too!
But there's a difference.
After the Enterprise-A, the name has only passed on when there's been a new ship class.
Original Enterprise - Constitution Class
1701-A - Constitution II (depends on one's canon).
1701-B - Excelsior
1701-C - Nebula
1701-D - Galaxy
1701-E - Soverign
Well, the other difference is that each time they came up with a new ENT for a tv show or movie, they clearly wanted to distinguish it from its predecessors or successors, either to make it easier to see what was going on onscreen (eg Yesterday's Enterprise) or simply to sell more toys.
Whereas in DS9, by the time the original Defiant bought it, the show was cash-strapped and was re-using stock footage for some space scenes. They couldn't afford a whole new ship to design action sequences or computer programmes around, which is why the next Defiant even had an identical registry; 'in-universe', we were told that they got a special dispensation because the original ship had perished before its time, after being so invaluable to the war effort.
In-universe, I also like Timo's
explanation that each ENT lasted so long that by the time it was destroyed or decommissioned, it was time for a new class of ship.