You are wrong. Of'course there is L1 and L2 cache content replacement after every context switch. It has some performance pros and some cons. However at the end there are 25% SMT2 gain. And approximately 15% SMT4 gain.
You're going to need to provide a source for your assertion.
You probably don't understand what Amdahl's law mean. This is related to CFD, FEM and other math models of physics where parallelization is limited. But I was talking specifically about servers with HIGH number of threads (web servers, SQL, etc.) where SMT4 gives performance advantage. AMD knows how many customers might benefit from SMT4 and if their analysis shows good profit I'm sure they will bring it in Zen 3 (as Sparc and IBM did).
SMT4 may be beneficial for servers. But since Zen3 chiplets will be used in server, HEDT, and mainstream, and SMT4 will require added die space from a redesigned and expanded front-end, one of two things would need to be true:
1) AMD are splitting Zen3 chiplets into server design and mainstream/HEDT design
or
2) AMD are willing to decrease yields on chiplets for a feature that will be disabled (SMT2 only) on mainstream/HEDT systems, in order to see a minor server gain at the expense of increased power expenditure and heat production
I don't see either of those being true. Hence I will stick with the reality that SMT4 is a pipedream for Zen3.
And BTW those 128 cores of Cortex A77 (106% IPC of Skylake according SPECint2006) will be much faster than 64c/128t x86 CPU. Skylake with SMT2 gain +20% (120% total), so 60% per thread. And ARM A77 has 106% already, no SMT, so it's 60% vs. 106%.... that's almost double the performance per thread. ARM becomes more and more serious threat for x86 world (Amazor Graviton2, Nuvia, Cavium etc.). AMD has to evolve fast and bring innovations such as SMT4 as soon as possible because this server specific path ARM's Cortex cores cannot follow right know. Next new uarch for SMT4 will be Zen5 in 2022 and this might be too late. In 2022 ARM will have 25% laptops and 5-10% servers.
This is just speculation. ARM may have 25% marketshare in 2022 --- but only on the back of Apple if they do end up switching to ARM-based processors (which are not Cortex-based, mind you).
I doubt AMD is worried about Apple switching from Intel to ARM, since Apple's Mac laptop shipments are growing at a whopping 1% per year. AMD cares more about digging into Intel's marketshare among Dell, HP, and Lenovo laptops, who combined ship 65% of all laptops. ARM isn't going to make major inroads into the corporate laptop market (50% of laptop sales last year) since Windows ARM sucks and it's not going to be ready for corporate deployment in 2022, or realistically even 2025.
Dell - does not offer ANY ARM-based laptops on their online store, even their Chromebook is Intel-based
HP - does not offer ANY ARM-based laptops on their online store, even their Chromebook is Intel-based
Lenovo - has some ARM offerings, not many, certainly nothing enterprise-worthy
Even IF Apple moves to ARM, it's not Cortex, it's just ARMv8 based, Apple's software may not even be compatible with other ARM-based processors, and even then, the only market ARM-proper (Cortex-based) is going to keep making significant inroads into by 2022 is the low-margin cheap laptop area. Even then, it may have difficulty hitting 25% even WITH Apple's conversion.