Actually, the SPARC T4 and T5 systems are the best selling SPARC servers ever by volume. Since SPARC T4 introduced the concept of dynamic threading and critical thread support in Solaris, the poor single-threaded performance of the past T1-T3 CPUs is no longer an issue. There are plenty of public benchmarks that demonstrate how the T4/T5 perform and scale better than the Intel or AMD CPUs. Sun was the first to really embrace SoC (System on a Chip) in a commercial processor. Modern SPARC CPUs have PCIe controllers, memory controllers, cryptographic engines, coherence links for SMP, etc. all built into the CPU. Intel, AMD, and even IBM are all playing catch up to Oracle and Fujitsu on this front. Intel has relied too much on clock frequencies and this is why their performance has only improved ~15% if that. Meanwhile, SPARC performance has improved on average at least 2X with each generation since the acquisition. Oracle has done some great things with the investment in SPARC and Solaris.
Being able to scale up performance linearly to 32 CPU sockets on an SPARC M6 or 64 CPU sockets on the M10-4S is a pretty big accomplishment. Both of those systems scale up to 32TBs of RAM and a boat load of PCIe slots. They can be broken up into physical or logical domains for consolidation or run as a single system. Can't do that with x86 :) Those large systems are used by banks, e-commerce sites, governments, healthcare, insurance, etc companies who need the compute power on their back end systems (ERP, CRM, Financials, etc.). The T4/T5 servers scale up to 8 sockets and are used by companies to host web services, middleware, databases, etc. Most of the enterprise software out there is based on Java and is highly threaded, so they scale really well on SPARC. But with the T4/T5, the heavy single threaded apps are now performing well on SPARC.
The challenge for server manufacturers in the UNIX market (Oracle, IBM, HP) is that you don't need as many servers to replace old servers anymore. It's very easy to replaced an E25k with a small T4 server. It's not uncommon to see Solaris and AIX shops doing some extreme consolidations. The number of OS instances has not decreased in many sites, if anything they have gone up. The difference is that the number of physical servers had gone down significantly thanks to virtualization. This is the same problem that HP, Dell, and IBM have been struggling with since the introduction of VMware. I've been on many projects over the past 6 years where sites that had hundreds of UNIX servers have consolidated to a few racks of modern SPARC and Power servers. You just don't need as many physical servers anymore.
VMware, Linux, and x86 has been an impact, but if you look at what applications have moved in that direction, it's not the bulk of the business critical systems. It's the low hanging fruit which takes up the majority of the traditional data center (dev/test servers, web servers, IDM, middleware, print services, VDI, etc.). Things like CIFS/NFS shares for home directories and applications that use to be on UNIX and Windows servers have by far been replaced by NAS appliances. But when you get to the mission critical systems, where the money is really made in a business, it's usually going to be on a UNIX server or a mainframe.
You would be surprised to learn how many of the services you take advantage of in everyday life are back-ended and in some cases front-ended by SPARC and Power servers. Some of the major ones are banks and e-commerce sites all of us use every day.
FYI.. SPARC v9 is the ISA version, not the CPU version or model number :p