The following is from Matt Freestone, a product manager at Adobe who is responsible for ExcelClient and ReportBuilder:
We have no short-term plans to discontinue support for ExcelClient. If in the future, should we determine to discontinue support for ExcelClient we will provide notice to our customers well in advance so that they can make plans accordingly. In the mean time, we support both tools. Development of new or advanced functionality will likely center around ReportBuilder, but we continue to provide bug fixes for ExcelClient.