plus, when a generic developer writes the custom code it eventually evolves into an endless cycle of implementing Excel features the domain expert is use to having a click or two away.
Plus, in my observation, there is often a dismissive attitude towards Excel among the generic developers which blinds them to the risk of this “re-implementing Excel’s features” cycle.
I worked on a team that helped business users who had "outgrown excel", i.e., they had hung themselves with the rope Excel provides. Almost always their scaling problems were solved simply by better Excel practices: better management of the calculation mode; setting the RTD throttle interval to 1-2 seconds; replacing Bloomberg's streaming data function (BDP), with the native alternative {=RTD("BLOOMBERG.RTD...)}; optimizing the division of labor between what is done on the sheet with formulas versus with vba\xll code; meta programming, i.e., creating all or part of your calc sheets and formulas with code so that you get the understandability and observability of Excel formulas while avoiding the things that are hard to do with formulas, such as grouping, joining, filtering, looping; making 3rd-party add-ins workbook-specific such that they're not always on (many of these add-ins listen to application-level events like selection_change and on_calculation which can diminish the performance of all open Excel models, even the ones that don't use that add-in).
I worked on a team that helped business users who had "outgrown excel", i.e., they had hung themselves with the rope Excel provides. Almost always their scaling problems were solved simply by better Excel practices: better management of the calculation mode; setting the RTD throttle interval to 1-2 seconds; replacing Bloomberg's streaming data function (BDP), with the native alternative {=RTD("BLOOMBERG.RTD...)}; optimizing the division of labor between what is done on the sheet with formulas versus with vba\xll code; meta programming, i.e., creating all or part of your calc sheets and formulas with code so that you get the understandability and observability of Excel formulas while avoiding the things that are hard to do with formulas, such as grouping, joining, filtering, looping; making 3rd-party add-ins workbook-specific such that they're not always on (many of these add-ins listen to application-level events like selection_change and on_calculation which can diminish the performance of all open Excel models, even the ones that don't use that add-in).