Different cell lines exist for different reasons, depending on the cell type and their properties they are useful for different experiments.
HEK-293 - Embryonic Stem Cells - Female
WI-38 - Fibroblasts derived from lung tissue - 3 month old female
MRC-5 - Fibroblasts derived from lung tissue - 14 week old male
Embryonic stem cells are totipotent, meaning they have the ability to differentiate into any other cell type.
Unfortunately to my knowledge we cannot yet generate totipotent cell lines. The cell line HEK-293 (or any embryonic stem cell line) are useful for studies that require the use of embryonic/totipotent stem cells.
The other cell lines - WI-38 & MRC-5 may be replaced by others quite easily however it may be the age of these cell lines that makes them attractive for scientific use. Scientists collaborate - when a cell line is able to 'do something' others may want to use it for their experiments too, and that is why particular cell lines are popular.
The ethical quandary of aborted fetus stem cells or stem cells sources from non-aborted fetuses. Yes it is possible to get a type of stem cell from E.g. Cord Blood (Blood from the umbilical cord of a baby) or Amniocentesis. But not embryonic/totipotent stem cells.
"The stem cells found in cord blood are often confused with embryonic stem cells - unlike embryonic stem cells, cord blood stem cells are all types of adult stem cells, are lineage restricted and are not pluripotent."
To avoid ethical concerns induced pluripotent stem cells (IPSCs) may be used. IPSCs have the ability to differentiate into almost any other cell type, it is easy to generate a large number of cells, they are great for modeling disease & have a huge
transplantation potential. IPSCs are pluripotent, not totipotent. The image below outlines how IPSCs are generated, how they avoid ethical concerns and, IPSCs potential to be transplanted back into the patient. More information on how and why IPSCs were discovered can be found here.