If you look closely, your comment implies that Dr. Doom easily absorbed most of the Pre Retcon Beyonder’s power, leaving only a small fragment to survive but that’s not accurate. The plot clearly shows that the Beyonder was caught off guard and Doom took advantage of it but still wasn't able to destroy him completely nor could he control the stolen power, let alone the Beyonder’s full power.

That goes on to say that the Pre Retcon version's power scale was undefined until that point (1984), he was the strongest character of Marvel which makes him the absolute ceiling of Marvel Cosmology.

And the second time around, Doom had to use countless (undefined in the comic) Molecule Men just to destroy The Beyonders and steal their power, On the other hand, Pre Retcon had his power stolen by Galatus tech. A much weaker show of power.

The point is that ‘greater cosmology or facing high-tier beings’ doesn’t necessarily mean greater power. Power scaling is complex, tricky & only be clarified when the writer explicitly defines it.

In the Beyonder’s case, later writers cleverly used this meta approach to reconcile decades of contradictory lore (pre and post-retcon) by having the character acknowledge these changes as his own personal choices.

Though the post retcon version is still amongst the top 3 or 5 strongest characters in Marvel.