My grandparents intend to blend hot soups in addition to cold grapes. To them, glass feels more fragile and untrustworthy compared to stainless steel. So is stainless steel stouter?
They know that opacity is one disadvantage of Stainless Steel; they haven't seen a stainless steel container/jar that's transparent.
I'm surprised your grand parents did not have a glass blender before; they were and are still very common.
Glass jars are safe; I've not heard of reported problems with them; and have been in use for many, many years.
They are made with tempered glass and should not break during normal usage.
Also have a look at blender with plastic jars; maybe that could be a good solution.