You are correct - there could be other reasons not to develop a 5D5.Not exactly. A 5D Mark V might be shelved for a wide variety of reasons unrelated to your explanation.
The primary alternative reason I can think of is very simple: collapsing demand for DSLRs in the market place. According to CIPA, there was a 32% decline in DSLR sales in 2019. Obviously Canon is much more attuned to the market than any customer is, and they may decide that there is just not enough demand to warrant developing a 5D5. They may have even been working on a 5D5 but cancelled the project given what has been happening in the market. We simply don't know and may never know.
Hopefully we get some further information when the R5 is introduced.