Welds weakening the base metal is a myth that is disproven literally day 1 of any welding course. There are metals that don't like the heating and will weaken, but if the weld breaks before the base metal, someone fucked up real bad.

Of course shit welding can cause weakening of the material but thats true of everything. Anything that is worth welding that also is important will use metals that have strong welding properties that make the weld stronger than the base material.